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20-01-89/T\Pen01
This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.
Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.
| Tape/disk: | Home » CEEFAX disks » telesoftware12.adl |
| Filename: | 20-01-89/T\Pen01 |
| Read OK: | ✔ |
| File size: | 39DC bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
Using a light pen with the BBC microcomputer - by - Gordon Horsington
---------------------------------------------------------------------
Module 1. Introduction to light pen software
--------------------------------------------
This module will introduce the techniques needed to calculate the position
of the light pen in terms of the X and Y text coordinates.
The maximum resolution of a light pen is one 6845 CRTC character. In modes
0, 3, 4, 6 and 7 the displayed text characters correspond with the CRTC
characters and the maximum resolution of any BBC light pen is one
displayed text character. In modes 1, 2 and 5 the resolution is greater
than one displayed text character but the position can still be calculated
in terms of the X and Y text coordinates. It is also possible to calculate
the position of the light pen in terms of the X and Y graphical
coordinates and this will be covered in module 4.
A light pen will produce a positive going TTL pulse whenever the sensor
picks up light given out by the monitor screen. This TTL pulse is usually
called a video strobe pulse (VSP) and the timing of the VSP corresponds
with the position of the light pen on the screen. When the VSP falls back
from logic 1 to logic 0 a number which corresponds with the light pen
position will be latched into the (read only) registers 16 (high byte) and
17 (low byte) of the 6845 CRTC. These two registers form the 14 light pen
register. If the light pen has not been used the light pen register will
contain zero.
In order to read the contents of any CRTC register, the register address
must first be written into &FE00, the 6845 address register, and then the
selected register may be read from &FE01, the 6845 data register. Acorn
have provided two Osbyte calls which should be used when writing to or
reading from Sheila (ie, page &FE). These calls are Osbyte &97 (decimal
151) to write to Sheila and Osbyte &96 (decimal 150) to read from Sheila.
These Osbyte calls are used with the X register containing the offset
within Sheila and the Y register containing the byte to be written (Osbyte
&97). The machine code routine in the demonstration program OFFSET uses
these Osbyte calls to read the contents of the light pen register. Using
the Osbyte calls helps to ensure that OFFSET is Tube compatible. Peeking
and poking the memory locations directly would make it incompatible with a
second processor.
After reading the contents of the light pen register it is necessary to
subtract a correction factor from the number to take account of the
different screen start addresses in the various screen modes. The table in
figure 1 shows these correction factors in the column headed offset. These
offset values can only be used if the screen has not been scrolled. If the
screen is scrolled the position of the start of the screen will have to be
taken into account as well. One method of dealing with this problem will
be covered in module 3.
Mode offset width scale Text resolution
-----+--------+-------+-------+------------------
0 | &0606 | &50 | &00 | single character
1 | &0606 | &50 | &01 | half character
2 | &0606 | &50 | &02 | quarter character
3 | &0806 | &50 | &00 | single character
4 | &0B04 | &28 | &00 | single character
5 | &0B04 | &28 | &01 | half character
6 | &0C04 | &28 | &00 | single character
7 | &2808 | &28 | &00 | single character
-----+--------+-------+-------+------------------
Figure 1. The values of offset, width and scale for each screen mode
--------------------------------------------------------------------
The column headed width in figure 1 shows the number of 6845 characters
per row in each screen mode. This number is not necessarily the same as
the number of displayed characters and the 6845 'sees' &50 (decimal 80)
characters per row in modes 0 to 3, and &28 (decimal 40) characters per
row in modes 4 to 7. The 6845 characters per row is not always the same as
the displayed characters per row and for this reason the light pen
coordinates need to be calculated in terms of displayed text coordinates
rather than 6845 coordinates. The light pen position can be calculated in
terms of displayed text coordinates using:
Y coordinate = (light pen register - offset) DIV width
X coordinate = ((light pen register - offset) MOD width) DIV (2^scale)
The values of offset, width and scale are those appropriate to the screen
mode in use at the time. The reason why 2^scale (2 raised to the power of
scale) is used in the calculation of the X coordinate is to simplify the
machine code arithmetic used to produce the X coordinate. The number
assigned to the variable scale indicates how many times the CRTC X
coordinate has to be divided by 2 to give the text X coordinate.
The machine code routine in the program OFFSET performs these calculations
in any screen mode leaving the results in &70 (X coordinate) and &71 (Y
coordinate). Although zero page will be used to store the results in all
the light pen modules any 3 bytes would do (&73 is used as workspace
during the calculation). Using zero page has the advantage of speeding up
the calculation.
The range of the X and Y coordinates made available depends on the screen
mode and corresponds with the values of X and Y used by the BASIC TAB(x,y)
command. The range of values are shown in figure 2.
Mode X (?&70) Y (?&71)
------+----------+----------
0 | 0 - 79 | 0 - 31
1 | 0 - 39 | 0 - 31
2 | 0 - 19 | 0 - 31
3 | 0 - 79 | 0 - 24
4 | 0 - 39 | 0 - 31
5 | 0 - 19 | 0 - 31
6 | 0 - 39 | 0 - 24
7 | 0 - 39 | 0 - 24
------+----------+----------
Figure 2. The range of X and Y coordinates in the various screen modes
----------------------------------------------------------------------
If all light pens and all computers were the same you could simply call
the machine code routine in the program OFFSET and read the X and Y text
coordinates from &70 and &71 every time you wanted the light pen position.
Unfortunately things are not quite that simple and there are problems
because different pens produce VSPs with slightly different timing and
there also seems to be a problem with Osbytes &96 and &97 with the BBC B
OS 1.20.
The best way of illustrating some of the problems with light pens is to
plug your light pen into the analogue port, switch on the computer and
chain the program OFFSET. This program uses the up and down cursor keys to
change the screen modes and the left and right cursor keys to adjust the
least significant byte of the offset.
If you move the light pen around the screen the program will display an
asterisk which follows the pen. You will probably find that the asterisk
either leads or, more likely, lags behind the pen. This can be adjusted
with the left and right cursor keys. The left cursor key will move the
asterisk to the left and the right cursor key will move it to the right.
Pressing the left cursor key adds 1 to the least significant byte of the
offset and pressing the right cursor key subtracts 1. This is known in
light pen jargon as trimming the offset. The current value of the least
significant byte is displayed by the program and you should use the
program to trim the offset in all the screen modes. I have found that the
value of the least significant byte of the offset is usually 1 or 2 less
than the theoretical value. You may well find that different values are
needed with your light pen. Make a note of the values your pen uses
because you will need to trim the offset in all the programs used to
illustrate the light pen modules. You can permanently alter the least
significant byte of the offsets by editing lines 1340 to 1410 of OFFSET.
The problem with Osbytes &96 and &97 is one I have only found when using a
light pen with OS 1.20. If you run the program on a BBC B with OS 1.20 you
could find that, every now and then, an asterisk is displayed in a random
position on the screen. This only happens occasionally but it can be
completely cured by not using the Osbyte calls. If you have this problem
then include lines 1220 to 1240 which have been commented out in OFFSET.
Lines 1220 to 1240 poke and peek the address and data registers directly
and doing this cures the problem but makes the program incompatible with
the Tube. A Tube compatible program which accesses the registers directly
will be used in module 3.
Another problem can be illustrated with OFFSET. If you hold the light pen
at the extreme left or right edge of the screen you could find that the
asterisk is displayed on the opposite edge. This problem can be minimised
by careful trimming and using a light shade on the pen but it cannot
always be completely eliminated. This should not be too much of a problem
when using the light pen to read text coordinates because, in practice,
you will rarely need to access these parts of the screen. It can, however,
cause a terrible mess if you use the light pen to draw on a graphics
screen and we will be coming back to look at this problem in module 4 when
drawing with a light pen is covered.
One last problem with light pens can be demonstrated if you position the
light pen in the lower right hand corner of the screen. The program will
print an asterisk and scroll the screen. You should try to scroll the
screen in this manner to discover the effect it has on the light pen text
coordinates. The solution to the error produced by screen scrolling will
be covered in module 3.
10 REM> OFFSET
20 DIM mcode &100
30 PROCmcode
40 mode=0
50 MODE mode
60 PROCnewmode
70 REPEAT
80 IF INKEY(-58) PROCup:MODE mode:PROCnewmode
90 IF INKEY(-42) PROCdown:MODE mode:PROCnewmode
100 IF INKEY(-26) PROCleft
110 IF INKEY(-122) PROCright
120 CALL mcode
130 IF (?xcoord<>oldx) OR (?ycoord<>oldy)
VDU31,oldx,oldy,32,31,?xcoord,?ycoord,42
140 oldx=?xcoord
150 oldy=?ycoord
160 UNTIL FALSE
170 :
180 DEFPROCleft
190 IF offsetlow?mode=12 ENDPROC
200 offsetlow?mode=offsetlow?mode+1
210 PRINTTAB(15,2);offsetlow?mode;" ";
220 PROCdelay
230 ENDPROC
240 :
250 DEFPROCright
260 IF offsetlow?mode=0 ENDPROC
270 offsetlow?mode=offsetlow?mode-1
280 PRINTTAB(15,2);offsetlow?mode;" ";
290 PROCdelay
300 ENDPROC
310 :
320 DEFPROCup
330 mode=mode+1
340 IF mode>7 mode=7
350 ENDPROC
360 :
370 DEFPROCdown
380 mode=mode-1
390 IF mode<0 mode=0
400 ENDPROC
410 :
420 DEFPROCnewmode
430 VDU23,1,0;0;0;0;
440 VDU19,0,6;0;
450 VDU19,1,4;0;
460 COLOUR1
470 PRINTTAB(3,1)"Mode ";mode
480 PRINTTAB(3,2)"offsetlow = ";offsetlow?mode
490 IF mode=7 PROCseven
500 CALL mcode
510 oldx=?xcoord
520 oldy=?ycoord
530 PROCdelay
540 ENDPROC
550 :
560 DEFPROCdelay
570 time=TIME+20
580 REPEAT UNTIL time<TIME
590 ENDPROC
600 :
610 DEFPROCseven
620 FOR line=0 TO 24
630 VDU31,0,line,134,157,132
640 NEXT
650 ENDPROC
660 :
670 DEFPROCmcode
680 xcoord=&70
690 ycoord=&71 :REM: &71-&72
700 osbyte=&FFF4
710 FORpass=0TO2STEP2
720 P%=mcode
730 [ OPT pass
740 LDA #&87 \ read screen mode
750 JSR osbyte
760 TYA \ screen mode in Y register
770 AND #&07 \ range 0-7
780 STA screen \ store screen mode
790 LDY #&10 \ light pen register, high byte
800 JSR readio \ read light pen register
810 CPY #&00 \ Y=0 if pen not used
820 BEQ return
830 STY ycoord+1 \ store high byte
840 LDY #&11 \ light pen register, low byte
850 JSR readio \ read light pen register
860 TYA \ low byte into A
870 LDY screen \ screen mode
880 CLD \ clear decimal flag
890 SEC \ prepare for subtraction
900 SBC offsetlow,Y \ subtract screen mode offset
910 STA ycoord \ low byte - offset
920 LDA ycoord+1 \ high byte
930 SBC offsethigh,Y \ subtract screen mode offset
940 STA ycoord+1 \ high byte - offset
950 LDA #&00 \ prepare for 16 bit division
960 STA xcoord \ clear low partial dividend
970 LDX #&10 \ set bit counter for 16 bit division
980 .next
990 ASL ycoord \ shift dividend/quotient left
1000 ROL ycoord+1 \ shift dividend/quotient left
1010 ROL xcoord \ shift bits into partial dividend
1020 LDA xcoord \ load partial dividend
1030 SEC \ prepare for subtraction
1040 SBC width,Y \ subtract divisor
1050 BCC done \ branch if dividend < divisor
1060 INC ycoord \ increment quotient
1070 STA xcoord \ save new partial dividend
1080 .done
1090 DEX \ decrement bit counter
1100 BNE next \ branch for 16 bits
1110 LDA scale,Y \ screen mode scale
1120 BEQ return \ branch if modes 0, 3, 4, 6 or 7
1130 TAX \ in mode1 and mode5 X=1, in mode2 X=2
1140 LDA xcoord
1150 .reduce
1160 LSR A \ horizontal position / 2
1170 DEX
1180 BNE reduce
1190 STA xcoord
1200 .return
1210 RTS \ return to BASIC
1220 .readio
1230 \ STY &FE00
1240 \ LDY &FE01
1250 \ RTS
1260 LDA #&97 \ write to sheila
1270 LDX #&00 \ offset 0 = &FE00
1280 JSR osbyte
1290 LDA #&96 \ read from sheila
1300 LDX #&01 \ offset 1 = &FE01
1310 JMP osbyte \ and return
1320 .screen
1330 EQUB &00 \ screen mode stored here
1340 .offsetlow
1350 EQUB &06 \ mode0, trimmed = 4
1360 EQUB &06 \ mode1, trimmed = 4
1370 EQUB &06 \ mode2, trimmed = 4
1380 EQUB &06 \ mode3, trimmed = 4
1390 EQUB &04 \ mode4, trimmed = 3
1400 EQUB &04 \ mode5, trimmed = 3
1410 EQUB &04 \ mode6, trimmed = 3
1420 EQUB &08 \ mode7, trimmed = 6
1430 .offsethigh
1440 EQUD &08060606 \ modes 3-0
1450 EQUD &280C0B0B \ modes 7-4
1460 .scale
1470 EQUD &00020100 \ modes 3-0
1480 EQUD &00000100 \ modes 7-4
1490 .width
1500 EQUD &50505050 \ modes 3-0
1510 EQUD &28282828 \ modes 7-4
1520 ]
1530 NEXT
1540 ENDPROC
00000000 55 73 69 6e 67 20 61 20 6c 69 67 68 74 20 70 65 |Using a light pe| 00000010 6e 20 77 69 74 68 20 74 68 65 20 42 42 43 20 6d |n with the BBC m| 00000020 69 63 72 6f 63 6f 6d 70 75 74 65 72 20 2d 20 62 |icrocomputer - b| 00000030 79 20 2d 20 47 6f 72 64 6f 6e 20 48 6f 72 73 69 |y - Gordon Horsi| 00000040 6e 67 74 6f 6e 0d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |ngton.----------| 00000050 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00000080 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 0d 0d 4d 6f 64 |-----------..Mod| 00000090 75 6c 65 20 31 2e 20 49 6e 74 72 6f 64 75 63 74 |ule 1. Introduct| 000000a0 69 6f 6e 20 74 6f 20 6c 69 67 68 74 20 70 65 6e |ion to light pen| 000000b0 20 73 6f 66 74 77 61 72 65 0d 2d 2d 2d 2d 2d 2d | software.------| 000000c0 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 000000e0 2d 2d 2d 2d 2d 2d 0d 0d 0d 54 68 69 73 20 6d 6f |------...This mo| 000000f0 64 75 6c 65 20 77 69 6c 6c 20 69 6e 74 72 6f 64 |dule will introd| 00000100 75 63 65 20 74 68 65 20 74 65 63 68 6e 69 71 75 |uce the techniqu| 00000110 65 73 20 6e 65 65 64 65 64 20 74 6f 20 63 61 6c |es needed to cal| 00000120 63 75 6c 61 74 65 20 74 68 65 20 70 6f 73 69 74 |culate the posit| 00000130 69 6f 6e 0d 6f 66 20 74 68 65 20 6c 69 67 68 74 |ion.of the light| 00000140 20 70 65 6e 20 69 6e 20 74 65 72 6d 73 20 6f 66 | pen in terms of| 00000150 20 74 68 65 20 58 20 61 6e 64 20 59 20 74 65 78 | the X and Y tex| 00000160 74 20 63 6f 6f 72 64 69 6e 61 74 65 73 2e 0d 0d |t coordinates...| 00000170 54 68 65 20 6d 61 78 69 6d 75 6d 20 72 65 73 6f |The maximum reso| 00000180 6c 75 74 69 6f 6e 20 6f 66 20 61 20 6c 69 67 68 |lution of a ligh| 00000190 74 20 70 65 6e 20 69 73 20 6f 6e 65 20 36 38 34 |t pen is one 684| 000001a0 35 20 43 52 54 43 20 63 68 61 72 61 63 74 65 72 |5 CRTC character| 000001b0 2e 20 49 6e 20 6d 6f 64 65 73 0d 30 2c 20 33 2c |. In modes.0, 3,| 000001c0 20 34 2c 20 36 20 61 6e 64 20 37 20 74 68 65 20 | 4, 6 and 7 the | 000001d0 64 69 73 70 6c 61 79 65 64 20 74 65 78 74 20 63 |displayed text c| 000001e0 68 61 72 61 63 74 65 72 73 20 63 6f 72 72 65 73 |haracters corres| 000001f0 70 6f 6e 64 20 77 69 74 68 20 74 68 65 20 43 52 |pond with the CR| 00000200 54 43 0d 63 68 61 72 61 63 74 65 72 73 20 61 6e |TC.characters an| 00000210 64 20 74 68 65 20 6d 61 78 69 6d 75 6d 20 72 65 |d the maximum re| 00000220 73 6f 6c 75 74 69 6f 6e 20 6f 66 20 61 6e 79 20 |solution of any | 00000230 42 42 43 20 6c 69 67 68 74 20 70 65 6e 20 69 73 |BBC light pen is| 00000240 20 6f 6e 65 0d 64 69 73 70 6c 61 79 65 64 20 74 | one.displayed t| 00000250 65 78 74 20 63 68 61 72 61 63 74 65 72 2e 20 49 |ext character. I| 00000260 6e 20 6d 6f 64 65 73 20 31 2c 20 32 20 61 6e 64 |n modes 1, 2 and| 00000270 20 35 20 74 68 65 20 72 65 73 6f 6c 75 74 69 6f | 5 the resolutio| 00000280 6e 20 69 73 20 67 72 65 61 74 65 72 0d 74 68 61 |n is greater.tha| 00000290 6e 20 6f 6e 65 20 64 69 73 70 6c 61 79 65 64 20 |n one displayed | 000002a0 74 65 78 74 20 63 68 61 72 61 63 74 65 72 20 62 |text character b| 000002b0 75 74 20 74 68 65 20 70 6f 73 69 74 69 6f 6e 20 |ut the position | 000002c0 63 61 6e 20 73 74 69 6c 6c 20 62 65 20 63 61 6c |can still be cal| 000002d0 63 75 6c 61 74 65 64 0d 69 6e 20 74 65 72 6d 73 |culated.in terms| 000002e0 20 6f 66 20 74 68 65 20 58 20 61 6e 64 20 59 20 | of the X and Y | 000002f0 74 65 78 74 20 63 6f 6f 72 64 69 6e 61 74 65 73 |text coordinates| 00000300 2e 20 49 74 20 69 73 20 61 6c 73 6f 20 70 6f 73 |. It is also pos| 00000310 73 69 62 6c 65 20 74 6f 20 63 61 6c 63 75 6c 61 |sible to calcula| 00000320 74 65 0d 74 68 65 20 70 6f 73 69 74 69 6f 6e 20 |te.the position | 00000330 6f 66 20 74 68 65 20 6c 69 67 68 74 20 70 65 6e |of the light pen| 00000340 20 69 6e 20 74 65 72 6d 73 20 6f 66 20 74 68 65 | in terms of the| 00000350 20 58 20 61 6e 64 20 59 20 67 72 61 70 68 69 63 | X and Y graphic| 00000360 61 6c 0d 63 6f 6f 72 64 69 6e 61 74 65 73 20 61 |al.coordinates a| 00000370 6e 64 20 74 68 69 73 20 77 69 6c 6c 20 62 65 20 |nd this will be | 00000380 63 6f 76 65 72 65 64 20 69 6e 20 6d 6f 64 75 6c |covered in modul| 00000390 65 20 34 2e 0d 0d 41 20 6c 69 67 68 74 20 70 65 |e 4...A light pe| 000003a0 6e 20 77 69 6c 6c 20 70 72 6f 64 75 63 65 20 61 |n will produce a| 000003b0 20 70 6f 73 69 74 69 76 65 20 67 6f 69 6e 67 20 | positive going | 000003c0 54 54 4c 20 70 75 6c 73 65 20 77 68 65 6e 65 76 |TTL pulse whenev| 000003d0 65 72 20 74 68 65 20 73 65 6e 73 6f 72 0d 70 69 |er the sensor.pi| 000003e0 63 6b 73 20 75 70 20 6c 69 67 68 74 20 67 69 76 |cks up light giv| 000003f0 65 6e 20 6f 75 74 20 62 79 20 74 68 65 20 6d 6f |en out by the mo| 00000400 6e 69 74 6f 72 20 73 63 72 65 65 6e 2e 20 54 68 |nitor screen. Th| 00000410 69 73 20 54 54 4c 20 70 75 6c 73 65 20 69 73 20 |is TTL pulse is | 00000420 75 73 75 61 6c 6c 79 0d 63 61 6c 6c 65 64 20 61 |usually.called a| 00000430 20 76 69 64 65 6f 20 73 74 72 6f 62 65 20 70 75 | video strobe pu| 00000440 6c 73 65 20 28 56 53 50 29 20 61 6e 64 20 74 68 |lse (VSP) and th| 00000450 65 20 74 69 6d 69 6e 67 20 6f 66 20 74 68 65 20 |e timing of the | 00000460 56 53 50 20 63 6f 72 72 65 73 70 6f 6e 64 73 0d |VSP corresponds.| 00000470 77 69 74 68 20 74 68 65 20 70 6f 73 69 74 69 6f |with the positio| 00000480 6e 20 6f 66 20 74 68 65 20 6c 69 67 68 74 20 70 |n of the light p| 00000490 65 6e 20 6f 6e 20 74 68 65 20 73 63 72 65 65 6e |en on the screen| 000004a0 2e 20 57 68 65 6e 20 74 68 65 20 56 53 50 20 66 |. When the VSP f| 000004b0 61 6c 6c 73 20 62 61 63 6b 0d 66 72 6f 6d 20 6c |alls back.from l| 000004c0 6f 67 69 63 20 31 20 74 6f 20 6c 6f 67 69 63 20 |ogic 1 to logic | 000004d0 30 20 61 20 6e 75 6d 62 65 72 20 77 68 69 63 68 |0 a number which| 000004e0 20 63 6f 72 72 65 73 70 6f 6e 64 73 20 77 69 74 | corresponds wit| 000004f0 68 20 74 68 65 20 6c 69 67 68 74 20 70 65 6e 0d |h the light pen.| 00000500 70 6f 73 69 74 69 6f 6e 20 77 69 6c 6c 20 62 65 |position will be| 00000510 20 6c 61 74 63 68 65 64 20 69 6e 74 6f 20 74 68 | latched into th| 00000520 65 20 28 72 65 61 64 20 6f 6e 6c 79 29 20 72 65 |e (read only) re| 00000530 67 69 73 74 65 72 73 20 31 36 20 28 68 69 67 68 |gisters 16 (high| 00000540 20 62 79 74 65 29 20 61 6e 64 0d 31 37 20 28 6c | byte) and.17 (l| 00000550 6f 77 20 62 79 74 65 29 20 6f 66 20 74 68 65 20 |ow byte) of the | 00000560 36 38 34 35 20 43 52 54 43 2e 20 54 68 65 73 65 |6845 CRTC. These| 00000570 20 74 77 6f 20 72 65 67 69 73 74 65 72 73 20 66 | two registers f| 00000580 6f 72 6d 20 74 68 65 20 31 34 20 6c 69 67 68 74 |orm the 14 light| 00000590 20 70 65 6e 0d 72 65 67 69 73 74 65 72 2e 20 49 | pen.register. I| 000005a0 66 20 74 68 65 20 6c 69 67 68 74 20 70 65 6e 20 |f the light pen | 000005b0 68 61 73 20 6e 6f 74 20 62 65 65 6e 20 75 73 65 |has not been use| 000005c0 64 20 74 68 65 20 6c 69 67 68 74 20 70 65 6e 20 |d the light pen | 000005d0 72 65 67 69 73 74 65 72 20 77 69 6c 6c 0d 63 6f |register will.co| 000005e0 6e 74 61 69 6e 20 7a 65 72 6f 2e 0d 0d 49 6e 20 |ntain zero...In | 000005f0 6f 72 64 65 72 20 74 6f 20 72 65 61 64 20 74 68 |order to read th| 00000600 65 20 63 6f 6e 74 65 6e 74 73 20 6f 66 20 61 6e |e contents of an| 00000610 79 20 43 52 54 43 20 72 65 67 69 73 74 65 72 2c |y CRTC register,| 00000620 20 74 68 65 20 72 65 67 69 73 74 65 72 20 61 64 | the register ad| 00000630 64 72 65 73 73 0d 6d 75 73 74 20 66 69 72 73 74 |dress.must first| 00000640 20 62 65 20 77 72 69 74 74 65 6e 20 69 6e 74 6f | be written into| 00000650 20 26 46 45 30 30 2c 20 74 68 65 20 36 38 34 35 | &FE00, the 6845| 00000660 20 61 64 64 72 65 73 73 20 72 65 67 69 73 74 65 | address registe| 00000670 72 2c 20 61 6e 64 20 74 68 65 6e 20 74 68 65 0d |r, and then the.| 00000680 73 65 6c 65 63 74 65 64 20 72 65 67 69 73 74 65 |selected registe| 00000690 72 20 6d 61 79 20 62 65 20 72 65 61 64 20 66 72 |r may be read fr| 000006a0 6f 6d 20 26 46 45 30 31 2c 20 74 68 65 20 36 38 |om &FE01, the 68| 000006b0 34 35 20 64 61 74 61 20 72 65 67 69 73 74 65 72 |45 data register| 000006c0 2e 20 41 63 6f 72 6e 0d 68 61 76 65 20 70 72 6f |. Acorn.have pro| 000006d0 76 69 64 65 64 20 74 77 6f 20 4f 73 62 79 74 65 |vided two Osbyte| 000006e0 20 63 61 6c 6c 73 20 77 68 69 63 68 20 73 68 6f | calls which sho| 000006f0 75 6c 64 20 62 65 20 75 73 65 64 20 77 68 65 6e |uld be used when| 00000700 20 77 72 69 74 69 6e 67 20 74 6f 20 6f 72 0d 72 | writing to or.r| 00000710 65 61 64 69 6e 67 20 66 72 6f 6d 20 53 68 65 69 |eading from Shei| 00000720 6c 61 20 28 69 65 2c 20 70 61 67 65 20 26 46 45 |la (ie, page &FE| 00000730 29 2e 20 54 68 65 73 65 20 63 61 6c 6c 73 20 61 |). These calls a| 00000740 72 65 20 4f 73 62 79 74 65 20 26 39 37 20 28 64 |re Osbyte &97 (d| 00000750 65 63 69 6d 61 6c 0d 31 35 31 29 20 74 6f 20 77 |ecimal.151) to w| 00000760 72 69 74 65 20 74 6f 20 53 68 65 69 6c 61 20 61 |rite to Sheila a| 00000770 6e 64 20 4f 73 62 79 74 65 20 26 39 36 20 28 64 |nd Osbyte &96 (d| 00000780 65 63 69 6d 61 6c 20 31 35 30 29 20 74 6f 20 72 |ecimal 150) to r| 00000790 65 61 64 20 66 72 6f 6d 20 53 68 65 69 6c 61 2e |ead from Sheila.| 000007a0 0d 54 68 65 73 65 20 4f 73 62 79 74 65 20 63 61 |.These Osbyte ca| 000007b0 6c 6c 73 20 61 72 65 20 75 73 65 64 20 77 69 74 |lls are used wit| 000007c0 68 20 74 68 65 20 58 20 72 65 67 69 73 74 65 72 |h the X register| 000007d0 20 63 6f 6e 74 61 69 6e 69 6e 67 20 74 68 65 20 | containing the | 000007e0 6f 66 66 73 65 74 0d 77 69 74 68 69 6e 20 53 68 |offset.within Sh| 000007f0 65 69 6c 61 20 61 6e 64 20 74 68 65 20 59 20 72 |eila and the Y r| 00000800 65 67 69 73 74 65 72 20 63 6f 6e 74 61 69 6e 69 |egister containi| 00000810 6e 67 20 74 68 65 20 62 79 74 65 20 74 6f 20 62 |ng the byte to b| 00000820 65 20 77 72 69 74 74 65 6e 20 28 4f 73 62 79 74 |e written (Osbyt| 00000830 65 0d 26 39 37 29 2e 20 54 68 65 20 6d 61 63 68 |e.&97). 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The table in| 00000a40 0d 66 69 67 75 72 65 20 31 20 73 68 6f 77 73 20 |.figure 1 shows | 00000a50 74 68 65 73 65 20 63 6f 72 72 65 63 74 69 6f 6e |these correction| 00000a60 20 66 61 63 74 6f 72 73 20 69 6e 20 74 68 65 20 | factors in the | 00000a70 63 6f 6c 75 6d 6e 20 68 65 61 64 65 64 20 6f 66 |column headed of| 00000a80 66 73 65 74 2e 20 54 68 65 73 65 0d 6f 66 66 73 |fset. These.offs| 00000a90 65 74 20 76 61 6c 75 65 73 20 63 61 6e 20 6f 6e |et values can on| 00000aa0 6c 79 20 62 65 20 75 73 65 64 20 69 66 20 74 68 |ly be used if th| 00000ab0 65 20 73 63 72 65 65 6e 20 68 61 73 20 6e 6f 74 |e screen has not| 00000ac0 20 62 65 65 6e 20 73 63 72 6f 6c 6c 65 64 2e 20 | been scrolled. | 00000ad0 49 66 20 74 68 65 0d 73 63 72 65 65 6e 20 69 73 |If the.screen is| 00000ae0 20 73 63 72 6f 6c 6c 65 64 20 74 68 65 20 70 6f | scrolled the po| 00000af0 73 69 74 69 6f 6e 20 6f 66 20 74 68 65 20 73 74 |sition of the st| 00000b00 61 72 74 20 6f 66 20 74 68 65 20 73 63 72 65 65 |art of the scree| 00000b10 6e 20 77 69 6c 6c 20 68 61 76 65 20 74 6f 20 62 |n will have to b| 00000b20 65 0d 74 61 6b 65 6e 20 69 6e 74 6f 20 61 63 63 |e.taken into acc| 00000b30 6f 75 6e 74 20 61 73 20 77 65 6c 6c 2e 20 4f 6e |ount as well. 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The values of| 00000e50 20 6f 66 66 73 65 74 2c 20 77 69 64 74 68 20 61 | offset, width a| 00000e60 6e 64 20 73 63 61 6c 65 20 66 6f 72 20 65 61 63 |nd scale for eac| 00000e70 68 20 73 63 72 65 65 6e 20 6d 6f 64 65 0d 2d 2d |h screen mode.--| 00000e80 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d |----------------| * 00000ec0 2d 2d 0d 0d 0d 54 68 65 20 63 6f 6c 75 6d 6e 20 |--...The column | 00000ed0 68 65 61 64 65 64 20 77 69 64 74 68 20 69 6e 20 |headed width in | 00000ee0 66 69 67 75 72 65 20 31 20 73 68 6f 77 73 20 74 |figure 1 shows t| 00000ef0 68 65 20 6e 75 6d 62 65 72 20 6f 66 20 36 38 34 |he number of 684| 00000f00 35 20 63 68 61 72 61 63 74 65 72 73 0d 70 65 72 |5 characters.per| 00000f10 20 72 6f 77 20 69 6e 20 65 61 63 68 20 73 63 72 | row in each scr| 00000f20 65 65 6e 20 6d 6f 64 65 2e 20 54 68 69 73 20 6e |een mode. This n| 00000f30 75 6d 62 65 72 20 69 73 20 6e 6f 74 20 6e 65 63 |umber is not nec| 00000f40 65 73 73 61 72 69 6c 79 20 74 68 65 20 73 61 6d |essarily the sam| 00000f50 65 20 61 73 0d 74 68 65 20 6e 75 6d 62 65 72 20 |e as.the number | 00000f60 6f 66 20 64 69 73 70 6c 61 79 65 64 20 63 68 61 |of displayed cha| 00000f70 72 61 63 74 65 72 73 20 61 6e 64 20 74 68 65 20 |racters and the | 00000f80 36 38 34 35 20 27 73 65 65 73 27 20 26 35 30 20 |6845 'sees' &50 | 00000f90 28 64 65 63 69 6d 61 6c 20 38 30 29 0d 63 68 61 |(decimal 80).cha| 00000fa0 72 61 63 74 65 72 73 20 70 65 72 20 72 6f 77 20 |racters per row | 00000fb0 69 6e 20 6d 6f 64 65 73 20 30 20 74 6f 20 33 2c |in modes 0 to 3,| 00000fc0 20 61 6e 64 20 26 32 38 20 28 64 65 63 69 6d 61 | and &28 (decima| 00000fd0 6c 20 34 30 29 20 63 68 61 72 61 63 74 65 72 73 |l 40) characters| 00000fe0 20 70 65 72 0d 72 6f 77 20 69 6e 20 6d 6f 64 65 | per.row in mode| 00000ff0 73 20 34 20 74 6f 20 37 2e 20 54 68 65 20 36 38 |s 4 to 7. 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The program wi| 000025a0 6c 6c 0d 70 72 69 6e 74 20 61 6e 20 61 73 74 65 |ll.print an aste| 000025b0 72 69 73 6b 20 61 6e 64 20 73 63 72 6f 6c 6c 20 |risk and scroll | 000025c0 74 68 65 20 73 63 72 65 65 6e 2e 20 59 6f 75 20 |the screen. You | 000025d0 73 68 6f 75 6c 64 20 74 72 79 20 74 6f 20 73 63 |should try to sc| 000025e0 72 6f 6c 6c 20 74 68 65 0d 73 63 72 65 65 6e 20 |roll the.screen | 000025f0 69 6e 20 74 68 69 73 20 6d 61 6e 6e 65 72 20 74 |in this manner t| 00002600 6f 20 64 69 73 63 6f 76 65 72 20 74 68 65 20 65 |o discover the e| 00002610 66 66 65 63 74 20 69 74 20 68 61 73 20 6f 6e 20 |ffect it has on | 00002620 74 68 65 20 6c 69 67 68 74 20 70 65 6e 20 74 65 |the light pen te| 00002630 78 74 0d 63 6f 6f 72 64 69 6e 61 74 65 73 2e 20 |xt.coordinates. | 00002640 54 68 65 20 73 6f 6c 75 74 69 6f 6e 20 74 6f 20 |The solution to | 00002650 74 68 65 20 65 72 72 6f 72 20 70 72 6f 64 75 63 |the error produc| 00002660 65 64 20 62 79 20 73 63 72 65 65 6e 20 73 63 72 |ed by screen scr| 00002670 6f 6c 6c 69 6e 67 20 77 69 6c 6c 0d 62 65 20 63 |olling will.be c| 00002680 6f 76 65 72 65 64 20 69 6e 20 6d 6f 64 75 6c 65 |overed in module| 00002690 20 33 2e 0d 0d 0d 20 20 20 31 30 20 52 45 4d 3e | 3.... 10 REM>| 000026a0 20 4f 46 46 53 45 54 0d 20 20 20 32 30 20 44 49 | OFFSET. 20 DI| 000026b0 4d 20 6d 63 6f 64 65 20 26 31 30 30 0d 20 20 20 |M mcode &100. | 000026c0 33 30 20 50 52 4f 43 6d 63 6f 64 65 0d 20 20 20 |30 PROCmcode. | 000026d0 34 30 20 6d 6f 64 65 3d 30 0d 20 20 20 35 30 20 |40 mode=0. 50 | 000026e0 4d 4f 44 45 20 6d 6f 64 65 0d 20 20 20 36 30 20 |MODE mode. 60 | 000026f0 50 52 4f 43 6e 65 77 6d 6f 64 65 0d 20 20 20 37 |PROCnewmode. 7| 00002700 30 20 52 45 50 45 41 54 0d 20 20 20 38 30 20 49 |0 REPEAT. 80 I| 00002710 46 20 49 4e 4b 45 59 28 2d 35 38 29 20 50 52 4f |F INKEY(-58) PRO| 00002720 43 75 70 3a 4d 4f 44 45 20 6d 6f 64 65 3a 50 52 |Cup:MODE mode:PR| 00002730 4f 43 6e 65 77 6d 6f 64 65 0d 20 20 20 39 30 20 |OCnewmode. 90 | 00002740 49 46 20 49 4e 4b 45 59 28 2d 34 32 29 20 50 52 |IF INKEY(-42) PR| 00002750 4f 43 64 6f 77 6e 3a 4d 4f 44 45 20 6d 6f 64 65 |OCdown:MODE mode| 00002760 3a 50 52 4f 43 6e 65 77 6d 6f 64 65 0d 20 20 31 |:PROCnewmode. 1| 00002770 30 30 20 49 46 20 49 4e 4b 45 59 28 2d 32 36 29 |00 IF INKEY(-26)| 00002780 20 50 52 4f 43 6c 65 66 74 0d 20 20 31 31 30 20 | PROCleft. 110 | 00002790 49 46 20 49 4e 4b 45 59 28 2d 31 32 32 29 20 50 |IF INKEY(-122) P| 000027a0 52 4f 43 72 69 67 68 74 0d 20 20 31 32 30 20 43 |ROCright. 120 C| 000027b0 41 4c 4c 20 6d 63 6f 64 65 0d 20 20 31 33 30 20 |ALL mcode. 130 | 000027c0 49 46 20 28 3f 78 63 6f 6f 72 64 3c 3e 6f 6c 64 |IF (?xcoord<>old| 000027d0 78 29 20 4f 52 20 28 3f 79 63 6f 6f 72 64 3c 3e |x) OR (?ycoord<>| 000027e0 6f 6c 64 79 29 0d 20 20 20 20 20 20 56 44 55 33 |oldy). VDU3| 000027f0 31 2c 6f 6c 64 78 2c 6f 6c 64 79 2c 33 32 2c 33 |1,oldx,oldy,32,3| 00002800 31 2c 3f 78 63 6f 6f 72 64 2c 3f 79 63 6f 6f 72 |1,?xcoord,?ycoor| 00002810 64 2c 34 32 0d 20 20 31 34 30 20 6f 6c 64 78 3d |d,42. 140 oldx=| 00002820 3f 78 63 6f 6f 72 64 0d 20 20 31 35 30 20 6f 6c |?xcoord. 150 ol| 00002830 64 79 3d 3f 79 63 6f 6f 72 64 0d 20 20 31 36 30 |dy=?ycoord. 160| 00002840 20 55 4e 54 49 4c 20 46 41 4c 53 45 0d 20 20 31 | UNTIL FALSE. 1| 00002850 37 30 20 3a 0d 20 20 31 38 30 20 44 45 46 50 52 |70 :. 180 DEFPR| 00002860 4f 43 6c 65 66 74 0d 20 20 31 39 30 20 49 46 20 |OCleft. 190 IF | 00002870 6f 66 66 73 65 74 6c 6f 77 3f 6d 6f 64 65 3d 31 |offsetlow?mode=1| 00002880 32 20 45 4e 44 50 52 4f 43 0d 20 20 32 30 30 20 |2 ENDPROC. 200 | 00002890 6f 66 66 73 65 74 6c 6f 77 3f 6d 6f 64 65 3d 6f |offsetlow?mode=o| 000028a0 66 66 73 65 74 6c 6f 77 3f 6d 6f 64 65 2b 31 0d |ffsetlow?mode+1.| 000028b0 20 20 32 31 30 20 50 52 49 4e 54 54 41 42 28 31 | 210 PRINTTAB(1| 000028c0 35 2c 32 29 3b 6f 66 66 73 65 74 6c 6f 77 3f 6d |5,2);offsetlow?m| 000028d0 6f 64 65 3b 22 20 22 3b 0d 20 20 32 32 30 20 50 |ode;" ";. 220 P| 000028e0 52 4f 43 64 65 6c 61 79 0d 20 20 32 33 30 20 45 |ROCdelay. 230 E| 000028f0 4e 44 50 52 4f 43 0d 20 20 32 34 30 20 3a 0d 20 |NDPROC. 240 :. | 00002900 20 32 35 30 20 44 45 46 50 52 4f 43 72 69 67 68 | 250 DEFPROCrigh| 00002910 74 0d 20 20 32 36 30 20 49 46 20 6f 66 66 73 65 |t. 260 IF offse| 00002920 74 6c 6f 77 3f 6d 6f 64 65 3d 30 20 45 4e 44 50 |tlow?mode=0 ENDP| 00002930 52 4f 43 0d 20 20 32 37 30 20 6f 66 66 73 65 74 |ROC. 270 offset| 00002940 6c 6f 77 3f 6d 6f 64 65 3d 6f 66 66 73 65 74 6c |low?mode=offsetl| 00002950 6f 77 3f 6d 6f 64 65 2d 31 0d 20 20 32 38 30 20 |ow?mode-1. 280 | 00002960 50 52 49 4e 54 54 41 42 28 31 35 2c 32 29 3b 6f |PRINTTAB(15,2);o| 00002970 66 66 73 65 74 6c 6f 77 3f 6d 6f 64 65 3b 22 20 |ffsetlow?mode;" | 00002980 22 3b 0d 20 20 32 39 30 20 50 52 4f 43 64 65 6c |";. 290 PROCdel| 00002990 61 79 0d 20 20 33 30 30 20 45 4e 44 50 52 4f 43 |ay. 300 ENDPROC| 000029a0 0d 20 20 33 31 30 20 3a 0d 20 20 33 32 30 20 44 |. 310 :. 320 D| 000029b0 45 46 50 52 4f 43 75 70 0d 20 20 33 33 30 20 6d |EFPROCup. 330 m| 000029c0 6f 64 65 3d 6d 6f 64 65 2b 31 0d 20 20 33 34 30 |ode=mode+1. 340| 000029d0 20 49 46 20 6d 6f 64 65 3e 37 20 6d 6f 64 65 3d | IF mode>7 mode=| 000029e0 37 0d 20 20 33 35 30 20 45 4e 44 50 52 4f 43 0d |7. 350 ENDPROC.| 000029f0 20 20 33 36 30 20 3a 0d 20 20 33 37 30 20 44 45 | 360 :. 370 DE| 00002a00 46 50 52 4f 43 64 6f 77 6e 0d 20 20 33 38 30 20 |FPROCdown. 380 | 00002a10 6d 6f 64 65 3d 6d 6f 64 65 2d 31 0d 20 20 33 39 |mode=mode-1. 39| 00002a20 30 20 49 46 20 6d 6f 64 65 3c 30 20 6d 6f 64 65 |0 IF mode<0 mode| 00002a30 3d 30 0d 20 20 34 30 30 20 45 4e 44 50 52 4f 43 |=0. 400 ENDPROC| 00002a40 0d 20 20 34 31 30 20 3a 0d 20 20 34 32 30 20 44 |. 410 :. 420 D| 00002a50 45 46 50 52 4f 43 6e 65 77 6d 6f 64 65 0d 20 20 |EFPROCnewmode. | 00002a60 34 33 30 20 56 44 55 32 33 2c 31 2c 30 3b 30 3b |430 VDU23,1,0;0;| 00002a70 30 3b 30 3b 0d 20 20 34 34 30 20 56 44 55 31 39 |0;0;. 440 VDU19| 00002a80 2c 30 2c 36 3b 30 3b 0d 20 20 34 35 30 20 56 44 |,0,6;0;. 450 VD| 00002a90 55 31 39 2c 31 2c 34 3b 30 3b 0d 20 20 34 36 30 |U19,1,4;0;. 460| 00002aa0 20 43 4f 4c 4f 55 52 31 0d 20 20 34 37 30 20 50 | COLOUR1. 470 P| 00002ab0 52 49 4e 54 54 41 42 28 33 2c 31 29 22 4d 6f 64 |RINTTAB(3,1)"Mod| 00002ac0 65 20 22 3b 6d 6f 64 65 0d 20 20 34 38 30 20 50 |e ";mode. 480 P| 00002ad0 52 49 4e 54 54 41 42 28 33 2c 32 29 22 6f 66 66 |RINTTAB(3,2)"off| 00002ae0 73 65 74 6c 6f 77 20 3d 20 22 3b 6f 66 66 73 65 |setlow = ";offse| 00002af0 74 6c 6f 77 3f 6d 6f 64 65 0d 20 20 34 39 30 20 |tlow?mode. 490 | 00002b00 49 46 20 6d 6f 64 65 3d 37 20 50 52 4f 43 73 65 |IF mode=7 PROCse| 00002b10 76 65 6e 0d 20 20 35 30 30 20 43 41 4c 4c 20 6d |ven. 500 CALL m| 00002b20 63 6f 64 65 0d 20 20 35 31 30 20 6f 6c 64 78 3d |code. 510 oldx=| 00002b30 3f 78 63 6f 6f 72 64 0d 20 20 35 32 30 20 6f 6c |?xcoord. 520 ol| 00002b40 64 79 3d 3f 79 63 6f 6f 72 64 0d 20 20 35 33 30 |dy=?ycoord. 530| 00002b50 20 50 52 4f 43 64 65 6c 61 79 0d 20 20 35 34 30 | PROCdelay. 540| 00002b60 20 45 4e 44 50 52 4f 43 0d 20 20 35 35 30 20 3a | ENDPROC. 550 :| 00002b70 0d 20 20 35 36 30 20 44 45 46 50 52 4f 43 64 65 |. 560 DEFPROCde| 00002b80 6c 61 79 0d 20 20 35 37 30 20 74 69 6d 65 3d 54 |lay. 570 time=T| 00002b90 49 4d 45 2b 32 30 0d 20 20 35 38 30 20 52 45 50 |IME+20. 580 REP| 00002ba0 45 41 54 20 55 4e 54 49 4c 20 74 69 6d 65 3c 54 |EAT UNTIL time<T| 00002bb0 49 4d 45 0d 20 20 35 39 30 20 45 4e 44 50 52 4f |IME. 590 ENDPRO| 00002bc0 43 0d 20 20 36 30 30 20 3a 0d 20 20 36 31 30 20 |C. 600 :. 610 | 00002bd0 44 45 46 50 52 4f 43 73 65 76 65 6e 0d 20 20 36 |DEFPROCseven. 6| 00002be0 32 30 20 46 4f 52 20 6c 69 6e 65 3d 30 20 54 4f |20 FOR line=0 TO| 00002bf0 20 32 34 0d 20 20 36 33 30 20 56 44 55 33 31 2c | 24. 630 VDU31,| 00002c00 30 2c 6c 69 6e 65 2c 31 33 34 2c 31 35 37 2c 31 |0,line,134,157,1| 00002c10 33 32 0d 20 20 36 34 30 20 4e 45 58 54 0d 20 20 |32. 640 NEXT. | 00002c20 36 35 30 20 45 4e 44 50 52 4f 43 0d 20 20 36 36 |650 ENDPROC. 66| 00002c30 30 20 3a 0d 20 20 36 37 30 20 44 45 46 50 52 4f |0 :. 670 DEFPRO| 00002c40 43 6d 63 6f 64 65 0d 20 20 36 38 30 20 78 63 6f |Cmcode. 680 xco| 00002c50 6f 72 64 3d 26 37 30 0d 20 20 36 39 30 20 79 63 |ord=&70. 690 yc| 00002c60 6f 6f 72 64 3d 26 37 31 20 3a 52 45 4d 3a 20 26 |oord=&71 :REM: &| 00002c70 37 31 2d 26 37 32 0d 20 20 37 30 30 20 6f 73 62 |71-&72. 700 osb| 00002c80 79 74 65 3d 26 46 46 46 34 0d 20 20 37 31 30 20 |yte=&FFF4. 710 | 00002c90 46 4f 52 70 61 73 73 3d 30 54 4f 32 53 54 45 50 |FORpass=0TO2STEP| 00002ca0 32 0d 20 20 37 32 30 20 50 25 3d 6d 63 6f 64 65 |2. 720 P%=mcode| 00002cb0 0d 20 20 37 33 30 20 5b 20 20 20 20 20 20 20 4f |. 730 [ O| 00002cc0 50 54 20 70 61 73 73 0d 20 20 37 34 30 20 20 20 |PT pass. 740 | 00002cd0 20 20 20 20 20 20 4c 44 41 20 23 26 38 37 20 20 | LDA #&87 | 00002ce0 20 20 20 20 5c 20 72 65 61 64 20 73 63 72 65 65 | \ read scree| 00002cf0 6e 20 6d 6f 64 65 0d 20 20 37 35 30 20 20 20 20 |n mode. 750 | 00002d00 20 20 20 20 20 4a 53 52 20 6f 73 62 79 74 65 0d | JSR osbyte.| 00002d10 20 20 37 36 30 20 20 20 20 20 20 20 20 20 54 59 | 760 TY| 00002d20 41 20 20 20 20 20 20 20 20 20 20 20 5c 20 73 63 |A \ sc| 00002d30 72 65 65 6e 20 6d 6f 64 65 20 69 6e 20 59 20 72 |reen mode in Y r| 00002d40 65 67 69 73 74 65 72 0d 20 20 37 37 30 20 20 20 |egister. 770 | 00002d50 20 20 20 20 20 20 41 4e 44 20 23 26 30 37 20 20 | AND #&07 | 00002d60 20 20 20 20 5c 20 72 61 6e 67 65 20 30 2d 37 0d | \ range 0-7.| 00002d70 20 20 37 38 30 20 20 20 20 20 20 20 20 20 53 54 | 780 ST| 00002d80 41 20 73 63 72 65 65 6e 20 20 20 20 5c 20 73 74 |A screen \ st| 00002d90 6f 72 65 20 73 63 72 65 65 6e 20 6d 6f 64 65 0d |ore screen mode.| 00002da0 20 20 37 39 30 20 20 20 20 20 20 20 20 20 4c 44 | 790 LD| 00002db0 59 20 23 26 31 30 20 20 20 20 20 20 5c 20 6c 69 |Y #&10 \ li| 00002dc0 67 68 74 20 70 65 6e 20 72 65 67 69 73 74 65 72 |ght pen 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20 64 | if dividend < d| 00003300 69 76 69 73 6f 72 0d 20 31 30 36 30 20 20 20 20 |ivisor. 1060 | 00003310 20 20 20 20 20 49 4e 43 20 79 63 6f 6f 72 64 20 | INC ycoord | 00003320 20 20 20 5c 20 69 6e 63 72 65 6d 65 6e 74 20 71 | \ increment q| 00003330 75 6f 74 69 65 6e 74 0d 20 31 30 37 30 20 20 20 |uotient. 1070 | 00003340 20 20 20 20 20 20 53 54 41 20 78 63 6f 6f 72 64 | STA xcoord| 00003350 20 20 20 20 5c 20 73 61 76 65 20 6e 65 77 20 70 | \ save new p| 00003360 61 72 74 69 61 6c 20 64 69 76 69 64 65 6e 64 0d |artial dividend.| 00003370 20 31 30 38 30 20 2e 64 6f 6e 65 0d 20 31 30 39 | 1080 .done. 109| 00003380 30 20 20 20 20 20 20 20 20 20 44 45 58 20 20 20 |0 DEX | 00003390 20 20 20 20 20 20 20 20 5c 20 64 65 63 72 65 6d | \ decrem| 000033a0 65 6e 74 20 62 69 74 20 63 6f 75 6e 74 65 72 0d |ent bit counter.| 000033b0 20 31 31 30 30 20 20 20 20 20 20 20 20 20 42 4e | 1100 BN| 000033c0 45 20 6e 65 78 74 20 20 20 20 20 20 5c 20 62 72 |E next \ br| 000033d0 61 6e 63 68 20 66 6f 72 20 31 36 20 62 69 74 73 |anch for 16 bits| 000033e0 0d 20 31 31 31 30 20 20 20 20 20 20 20 20 20 4c |. 1110 L| 000033f0 44 41 20 73 63 61 6c 65 2c 59 20 20 20 5c 20 73 |DA scale,Y \ s| 00003400 63 72 65 65 6e 20 6d 6f 64 65 20 73 63 61 6c 65 |creen mode scale| 00003410 0d 20 31 31 32 30 20 20 20 20 20 20 20 20 20 42 |. 1120 B| 00003420 45 51 20 72 65 74 75 72 6e 20 20 20 20 5c 20 62 |EQ return \ b| 00003430 72 61 6e 63 68 20 69 66 20 6d 6f 64 65 73 20 30 |ranch if modes 0| 00003440 2c 20 33 2c 20 34 2c 20 36 20 6f 72 20 37 0d 20 |, 3, 4, 6 or 7. | 00003450 31 31 33 30 20 20 20 20 20 20 20 20 20 54 41 58 |1130 TAX| 00003460 20 20 20 20 20 20 20 20 20 20 20 5c 20 69 6e 20 | \ in | 00003470 6d 6f 64 65 31 20 61 6e 64 20 6d 6f 64 65 35 20 |mode1 and mode5 | 00003480 58 3d 31 2c 20 69 6e 20 6d 6f 64 65 32 20 58 3d |X=1, in mode2 X=| 00003490 32 0d 20 31 31 34 30 20 20 20 20 20 20 20 20 20 |2. 1140 | 000034a0 4c 44 41 20 78 63 6f 6f 72 64 0d 20 31 31 35 30 |LDA xcoord. 1150| 000034b0 20 2e 72 65 64 75 63 65 0d 20 31 31 36 30 20 20 | .reduce. 1160 | 000034c0 20 20 20 20 20 20 20 4c 53 52 20 41 20 20 20 20 | LSR A | 000034d0 20 20 20 20 20 5c 20 68 6f 72 69 7a 6f 6e 74 61 | \ horizonta| 000034e0 6c 20 70 6f 73 69 74 69 6f 6e 20 2f 20 32 0d 20 |l position / 2. | 000034f0 31 31 37 30 20 20 20 20 20 20 20 20 20 44 45 58 |1170 DEX| 00003500 0d 20 31 31 38 30 20 20 20 20 20 20 20 20 20 42 |. 1180 B| 00003510 4e 45 20 72 65 64 75 63 65 0d 20 31 31 39 30 20 |NE reduce. 1190 | 00003520 20 20 20 20 20 20 20 20 53 54 41 20 78 63 6f 6f | STA xcoo| 00003530 72 64 0d 20 31 32 30 30 20 2e 72 65 74 75 72 6e |rd. 1200 .return| 00003540 0d 20 31 32 31 30 20 20 20 20 20 20 20 20 20 52 |. 1210 R| 00003550 54 53 20 20 20 20 20 20 20 20 20 20 20 5c 20 72 |TS \ r| 00003560 65 74 75 72 6e 20 74 6f 20 42 41 53 49 43 0d 20 |eturn to BASIC. | 00003570 31 32 32 30 20 2e 72 65 61 64 69 6f 0d 20 31 32 |1220 .readio. 12| 00003580 33 30 20 5c 20 20 20 20 20 20 20 53 54 59 20 26 |30 \ STY &| 00003590 46 45 30 30 0d 20 31 32 34 30 20 5c 20 20 20 20 |FE00. 1240 \ | 000035a0 20 20 20 4c 44 59 20 26 46 45 30 31 0d 20 31 32 | LDY &FE01. 12| 000035b0 35 30 20 5c 20 20 20 20 20 20 20 52 54 53 0d 20 |50 \ RTS. | 000035c0 31 32 36 30 20 20 20 20 20 20 20 20 20 4c 44 41 |1260 LDA| 000035d0 20 23 26 39 37 20 20 20 20 20 20 5c 20 77 72 69 | #&97 \ wri| 000035e0 74 65 20 74 6f 20 73 68 65 69 6c 61 0d 20 31 32 |te to sheila. 12| 000035f0 37 30 20 20 20 20 20 20 20 20 20 4c 44 58 20 23 |70 LDX #| 00003600 26 30 30 20 20 20 20 20 20 5c 20 6f 66 66 73 65 |&00 \ offse| 00003610 74 20 30 20 3d 20 26 46 45 30 30 0d 20 31 32 38 |t 0 = &FE00. 128| 00003620 30 20 20 20 20 20 20 20 20 20 4a 53 52 20 6f 73 |0 JSR os| 00003630 62 79 74 65 0d 20 31 32 39 30 20 20 20 20 20 20 |byte. 1290 | 00003640 20 20 20 4c 44 41 20 23 26 39 36 20 20 20 20 20 | LDA #&96 | 00003650 20 5c 20 72 65 61 64 20 66 72 6f 6d 20 73 68 65 | \ read from she| 00003660 69 6c 61 0d 20 31 33 30 30 20 20 20 20 20 20 20 |ila. 1300 | 00003670 20 20 4c 44 58 20 23 26 30 31 20 20 20 20 20 20 | LDX #&01 | 00003680 5c 20 6f 66 66 73 65 74 20 31 20 3d 20 26 46 45 |\ offset 1 = &FE| 00003690 30 31 0d 20 31 33 31 30 20 20 20 20 20 20 20 20 |01. 1310 | 000036a0 20 4a 4d 50 20 6f 73 62 79 74 65 20 20 20 20 5c | JMP osbyte \| 000036b0 20 61 6e 64 20 72 65 74 75 72 6e 0d 20 31 33 32 | and return. 132| 000036c0 30 20 2e 73 63 72 65 65 6e 0d 20 31 33 33 30 20 |0 .screen. 1330 | 000036d0 20 20 20 20 20 20 20 20 45 51 55 42 20 26 30 30 | EQUB &00| 000036e0 20 20 20 20 20 20 5c 20 73 63 72 65 65 6e 20 6d | \ screen m| 000036f0 6f 64 65 20 73 74 6f 72 65 64 20 68 65 72 65 0d |ode stored here.| 00003700 20 31 33 34 30 20 2e 6f 66 66 73 65 74 6c 6f 77 | 1340 .offsetlow| 00003710 0d 20 31 33 35 30 20 20 20 20 20 20 20 20 20 45 |. 1350 E| 00003720 51 55 42 20 26 30 36 20 20 20 20 20 20 5c 20 6d |QUB &06 \ m| 00003730 6f 64 65 30 2c 20 74 72 69 6d 6d 65 64 20 3d 20 |ode0, trimmed = | 00003740 34 0d 20 31 33 36 30 20 20 20 20 20 20 20 20 20 |4. 1360 | 00003750 45 51 55 42 20 26 30 36 20 20 20 20 20 20 5c 20 |EQUB &06 \ | 00003760 6d 6f 64 65 31 2c 20 74 72 69 6d 6d 65 64 20 3d |mode1, trimmed =| 00003770 20 34 0d 20 31 33 37 30 20 20 20 20 20 20 20 20 | 4. 1370 | 00003780 20 45 51 55 42 20 26 30 36 20 20 20 20 20 20 5c | EQUB &06 \| 00003790 20 6d 6f 64 65 32 2c 20 74 72 69 6d 6d 65 64 20 | mode2, trimmed | 000037a0 3d 20 34 0d 20 31 33 38 30 20 20 20 20 20 20 20 |= 4. 1380 | 000037b0 20 20 45 51 55 42 20 26 30 36 20 20 20 20 20 20 | EQUB &06 | 000037c0 5c 20 6d 6f 64 65 33 2c 20 74 72 69 6d 6d 65 64 |\ mode3, trimmed| 000037d0 20 3d 20 34 0d 20 31 33 39 30 20 20 20 20 20 20 | = 4. 1390 | 000037e0 20 20 20 45 51 55 42 20 26 30 34 20 20 20 20 20 | EQUB &04 | 000037f0 20 5c 20 6d 6f 64 65 34 2c 20 74 72 69 6d 6d 65 | \ mode4, trimme| 00003800 64 20 3d 20 33 0d 20 31 34 30 30 20 20 20 20 20 |d = 3. 1400 | 00003810 20 20 20 20 45 51 55 42 20 26 30 34 20 20 20 20 | EQUB &04 | 00003820 20 20 5c 20 6d 6f 64 65 35 2c 20 74 72 69 6d 6d | \ mode5, trimm| 00003830 65 64 20 3d 20 33 0d 20 31 34 31 30 20 20 20 20 |ed = 3. 1410 | 00003840 20 20 20 20 20 45 51 55 42 20 26 30 34 20 20 20 | EQUB &04 | 00003850 20 20 20 5c 20 6d 6f 64 65 36 2c 20 74 72 69 6d | \ mode6, trim| 00003860 6d 65 64 20 3d 20 33 0d 20 31 34 32 30 20 20 20 |med = 3. 1420 | 00003870 20 20 20 20 20 20 45 51 55 42 20 26 30 38 20 20 | EQUB &08 | 00003880 20 20 20 20 5c 20 6d 6f 64 65 37 2c 20 74 72 69 | \ mode7, tri| 00003890 6d 6d 65 64 20 3d 20 36 0d 20 31 34 33 30 20 2e |mmed = 6. 1430 .| 000038a0 6f 66 66 73 65 74 68 69 67 68 0d 20 31 34 34 30 |offsethigh. 1440| 000038b0 20 20 20 20 20 20 20 20 20 45 51 55 44 20 26 30 | EQUD &0| 000038c0 38 30 36 30 36 30 36 20 5c 20 6d 6f 64 65 73 20 |8060606 \ modes | 000038d0 33 2d 30 0d 20 31 34 35 30 20 20 20 20 20 20 20 |3-0. 1450 | 000038e0 20 20 45 51 55 44 20 26 32 38 30 43 30 42 30 42 | EQUD &280C0B0B| 000038f0 20 5c 20 6d 6f 64 65 73 20 37 2d 34 0d 20 31 34 | \ modes 7-4. 14| 00003900 36 30 20 2e 73 63 61 6c 65 0d 20 31 34 37 30 20 |60 .scale. 1470 | 00003910 20 20 20 20 20 20 20 20 45 51 55 44 20 26 30 30 | EQUD &00| 00003920 30 32 30 31 30 30 20 5c 20 6d 6f 64 65 73 20 33 |020100 \ modes 3| 00003930 2d 30 0d 20 31 34 38 30 20 20 20 20 20 20 20 20 |-0. 1480 | 00003940 20 45 51 55 44 20 26 30 30 30 30 30 31 30 30 20 | EQUD &00000100 | 00003950 5c 20 6d 6f 64 65 73 20 37 2d 34 0d 20 31 34 39 |\ modes 7-4. 149| 00003960 30 20 2e 77 69 64 74 68 0d 20 31 35 30 30 20 20 |0 .width. 1500 | 00003970 20 20 20 20 20 20 20 45 51 55 44 20 26 35 30 35 | EQUD &505| 00003980 30 35 30 35 30 20 5c 20 6d 6f 64 65 73 20 33 2d |05050 \ modes 3-| 00003990 30 0d 20 31 35 31 30 20 20 20 20 20 20 20 20 20 |0. 1510 | 000039a0 45 51 55 44 20 26 32 38 32 38 32 38 32 38 20 5c |EQUD &28282828 \| 000039b0 20 6d 6f 64 65 73 20 37 2d 34 0d 20 31 35 32 30 | modes 7-4. 1520| 000039c0 20 5d 0d 20 31 35 33 30 20 4e 45 58 54 0d 20 31 | ]. 1530 NEXT. 1| 000039d0 35 34 30 20 45 4e 44 50 52 4f 43 0d |540 ENDPROC.| 000039dc
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