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OS\BITS/T\OSB25
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 » telesoftware2.adl |
| Filename: | OS\BITS/T\OSB25 |
| Read OK: | ✔ |
| File size: | 148C bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
OSBITS - An Exploration of the BBC Micro at Machine Level
By Programmer
.........................................................
Part 25: Legal and Illegal Code
You often read about programming techniques with the BBC
Micro family that Acorn describe as 'illegal'. What does
this mean? Will the Bill be round to check on your code?
In fact the concept of legality in the BBC Micro OS relates
to using the OS in a certain way. The classic example is
writing to the screen. You can either use PLOT commands,
from machine code or from a high level language like BASIC,
or you can actually poke numbers into the screen memory to
light up individual pixels.
The main advantage of poking the screen is speed. The PLOT
routines are slow (at least when compared to poking the
screen). The disadvantages are three fold. Firstly the
address space in which the screen sits is not linear. You
don't start at the top left at address &3000 for a Mode 1
screen and progress across and down the screen evenly, like
a TV camera scans a picture. This non-linearity is caused
by the need to refresh the dynamic RAM in the micro.
To relate a pixel to its screen address you need a simple
formula, but one which includes much multiplying by &280.
So writing a program to poke the screen is not very
straightforward.
More importantly, and this might sound a little silly, but
you can not be sure where the screen will be in future
machines. Future-proofing your machine code is the
strongest argument for legal code and, in all cases except
arcade games, is pretty unchallengeable.
For example, in a BBC Model A the screen addresses are &4000
lower (where the modes exist) than on a Model B. When you
introduce shadow RAM in a B+ or a Master you find that you
might well be sure of the address but you will not know
whether it lies in the same bank of RAM in which your code
is being executed. This also means that you can not
directly poke the screen from a second processor without
using an OS call to cross the Tube. Future 32 bit BBC
Micros may well have the screen somewhere totally new again.
The third disadvantage to directly addressing the screen is
that changing your screen mode means you have to completely
rewrite your screen accessing code since the relation ship
between bytes and pixels is different for each mode.
As a general rule you should not poke the screen, or indeed
any other part of the OS or BASIC workspace memory. But if
your code could be considered to be an extension of the OS
(such as a mouse pointer implementation) you could claim to
have the right to do so. In this case you would have to
produce a different version of your code for each possible
machine used by your users. If you are just writing for
yourself then all these arguments are spurious anyway.
One interestingly blurred area of illegality (as defined by
Acorn) is VDU variables. The Master Reference Manual
(published by Acorn) actually gives you information to
enable you to access the VDU variables directly. These are
the bytes in memory which control mode definition, amongst
other things. However you are told to use OSBYTE calls to
read the variables and to read the memory address where they
start and then offset your writes from that. If, in future
machines, the table moves then your code should still work
if you used the relevant OSBYTE calls.
The BBC Micro operating system provides a wide range of
calls to help your programming. Acorn's micros do not have
a closed and undocumented OS like some home machines, and
even some mainframes. So, if you can, use the relevant OS
call.
One of the top programmers at Acorn, and the man who wrote
BBC BASIC, often says that the best test of legality is if
the code will run in a 6502 second processor and do its job
correctly. Although there are some legal tricks that work
only in the main micro this is a good rule of thumb.
The program in this module is a screen fill routine that
addresses the screen directly. To use it you must be
running in the micro itself, not a second processor, and you
must not have shadow mode engaged. It's not a particularly
elegant piece of code since most of it is taken up with
calculations to find the screen address that relates to a
graphics coordinate. It winds up being half the speed of
the CLG command in BASIC but much faster than using PLOTs to
do the same job, pixel by pixel.
When you have run the program, in whatever mode, you press
function key 4 and the routine will fill the screen in mode
1. The BASIC TIME facility will give a rough timing for the
procedure, which is about 1.1 seconds. Line 740 of the code
sets the byte to be poked to the screen. &00 will produce
black, &0F produces red, &F0 produces yellow and &FF
produces white. In mode 1 (as we have here) there are 2
bits per pixel to set colour and they group 4 pixels to a
byte like this:
Colour of pixel 1 - bits 7 and 3
Colour of pixel 2 - bits 6 and 2
Colour of pixel 3 - bits 5 and 1
Colour of pixel 4 - bits 4 and 0
That just about brings us to the end of this meander through
machine code on the BBC. The final module will offer some
suggestions for using a word processor (such as VIEW) to
edit your source code.
00000000 4f 53 42 49 54 53 20 2d 20 41 6e 20 45 78 70 6c |OSBITS - An Expl| 00000010 6f 72 61 74 69 6f 6e 20 6f 66 20 74 68 65 20 42 |oration of the B| 00000020 42 43 20 4d 69 63 72 6f 20 61 74 20 4d 61 63 68 |BC Micro at Mach| 00000030 69 6e 65 20 4c 65 76 65 6c 0d 0d 42 79 20 50 72 |ine Level..By Pr| 00000040 6f 67 72 61 6d 6d 65 72 0d 0d 2e 2e 2e 2e 2e 2e |ogrammer........| 00000050 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e |................| * 00000080 2e 2e 2e 0d 0d 0d 50 61 72 74 20 32 35 3a 20 4c |......Part 25: L| 00000090 65 67 61 6c 20 61 6e 64 20 49 6c 6c 65 67 61 6c |egal and Illegal| 000000a0 20 43 6f 64 65 0d 0d 0d 59 6f 75 20 6f 66 74 65 | Code...You ofte| 000000b0 6e 20 72 65 61 64 20 61 62 6f 75 74 20 70 72 6f |n read about pro| 000000c0 67 72 61 6d 6d 69 6e 67 20 74 65 63 68 6e 69 71 |gramming techniq| 000000d0 75 65 73 20 77 69 74 68 20 74 68 65 20 42 42 43 |ues with the BBC| 000000e0 0d 4d 69 63 72 6f 20 66 61 6d 69 6c 79 20 74 68 |.Micro family th| 000000f0 61 74 20 41 63 6f 72 6e 20 64 65 73 63 72 69 62 |at Acorn describ| 00000100 65 20 61 73 20 27 69 6c 6c 65 67 61 6c 27 2e 20 |e as 'illegal'. | 00000110 20 57 68 61 74 20 64 6f 65 73 0d 74 68 69 73 20 | What does.this | 00000120 6d 65 61 6e 3f 20 20 57 69 6c 6c 20 74 68 65 20 |mean? Will the | 00000130 42 69 6c 6c 20 62 65 20 72 6f 75 6e 64 20 74 6f |Bill be round to| 00000140 20 63 68 65 63 6b 20 6f 6e 20 79 6f 75 72 20 63 | check on your c| 00000150 6f 64 65 3f 0d 0d 49 6e 20 66 61 63 74 20 74 68 |ode?..In fact th| 00000160 65 20 63 6f 6e 63 65 70 74 20 6f 66 20 6c 65 67 |e concept of leg| 00000170 61 6c 69 74 79 20 69 6e 20 74 68 65 20 42 42 43 |ality in the BBC| 00000180 20 4d 69 63 72 6f 20 4f 53 20 72 65 6c 61 74 65 | Micro OS relate| 00000190 73 0d 74 6f 20 75 73 69 6e 67 20 74 68 65 20 4f |s.to using the O| 000001a0 53 20 69 6e 20 61 20 63 65 72 74 61 69 6e 20 77 |S in a certain w| 000001b0 61 79 2e 20 20 54 68 65 20 63 6c 61 73 73 69 63 |ay. The classic| 000001c0 20 65 78 61 6d 70 6c 65 20 69 73 0d 77 72 69 74 | example is.writ| 000001d0 69 6e 67 20 74 6f 20 74 68 65 20 73 63 72 65 65 |ing to the scree| 000001e0 6e 2e 20 20 59 6f 75 20 63 61 6e 20 65 69 74 68 |n. You can eith| 000001f0 65 72 20 75 73 65 20 50 4c 4f 54 20 63 6f 6d 6d |er use PLOT comm| 00000200 61 6e 64 73 2c 0d 66 72 6f 6d 20 6d 61 63 68 69 |ands,.from machi| 00000210 6e 65 20 63 6f 64 65 20 6f 72 20 66 72 6f 6d 20 |ne code or from | 00000220 61 20 68 69 67 68 20 6c 65 76 65 6c 20 6c 61 6e |a high level lan| 00000230 67 75 61 67 65 20 6c 69 6b 65 20 42 41 53 49 43 |guage like BASIC| 00000240 2c 0d 6f 72 20 79 6f 75 20 63 61 6e 20 61 63 74 |,.or you can act| 00000250 75 61 6c 6c 79 20 70 6f 6b 65 20 6e 75 6d 62 65 |ually poke numbe| 00000260 72 73 20 69 6e 74 6f 20 74 68 65 20 73 63 72 65 |rs into the scre| 00000270 65 6e 20 6d 65 6d 6f 72 79 20 74 6f 0d 6c 69 67 |en memory to.lig| 00000280 68 74 20 75 70 20 69 6e 64 69 76 69 64 75 61 6c |ht up individual| 00000290 20 70 69 78 65 6c 73 2e 0d 0d 54 68 65 20 6d 61 | pixels...The ma| 000002a0 69 6e 20 61 64 76 61 6e 74 61 67 65 20 6f 66 20 |in advantage of | 000002b0 70 6f 6b 69 6e 67 20 74 68 65 20 73 63 72 65 65 |poking the scree| 000002c0 6e 20 69 73 20 73 70 65 65 64 2e 20 20 54 68 65 |n is speed. The| 000002d0 20 50 4c 4f 54 0d 72 6f 75 74 69 6e 65 73 20 61 | PLOT.routines a| 000002e0 72 65 20 73 6c 6f 77 20 28 61 74 20 6c 65 61 73 |re slow (at leas| 000002f0 74 20 77 68 65 6e 20 63 6f 6d 70 61 72 65 64 20 |t when compared | 00000300 74 6f 20 70 6f 6b 69 6e 67 20 74 68 65 0d 73 63 |to poking the.sc| 00000310 72 65 65 6e 29 2e 20 20 54 68 65 20 64 69 73 61 |reen). The disa| 00000320 64 76 61 6e 74 61 67 65 73 20 61 72 65 20 74 68 |dvantages are th| 00000330 72 65 65 20 66 6f 6c 64 2e 20 20 46 69 72 73 74 |ree fold. First| 00000340 6c 79 20 74 68 65 0d 61 64 64 72 65 73 73 20 73 |ly the.address s| 00000350 70 61 63 65 20 69 6e 20 77 68 69 63 68 20 74 68 |pace in which th| 00000360 65 20 73 63 72 65 65 6e 20 73 69 74 73 20 69 73 |e screen sits is| 00000370 20 6e 6f 74 20 6c 69 6e 65 61 72 2e 20 20 59 6f | not linear. Yo| 00000380 75 0d 64 6f 6e 27 74 20 73 74 61 72 74 20 61 74 |u.don't start at| 00000390 20 74 68 65 20 74 6f 70 20 6c 65 66 74 20 61 74 | the top left at| 000003a0 20 61 64 64 72 65 73 73 20 26 33 30 30 30 20 66 | address &3000 f| 000003b0 6f 72 20 61 20 4d 6f 64 65 20 31 0d 73 63 72 65 |or a Mode 1.scre| 000003c0 65 6e 20 61 6e 64 20 70 72 6f 67 72 65 73 73 20 |en and progress | 000003d0 61 63 72 6f 73 73 20 61 6e 64 20 64 6f 77 6e 20 |across and down | 000003e0 74 68 65 20 73 63 72 65 65 6e 20 65 76 65 6e 6c |the screen evenl| 000003f0 79 2c 20 6c 69 6b 65 0d 61 20 54 56 20 63 61 6d |y, like.a TV cam| 00000400 65 72 61 20 73 63 61 6e 73 20 61 20 70 69 63 74 |era scans a pict| 00000410 75 72 65 2e 20 20 54 68 69 73 20 6e 6f 6e 2d 6c |ure. This non-l| 00000420 69 6e 65 61 72 69 74 79 20 69 73 20 63 61 75 73 |inearity is caus| 00000430 65 64 0d 62 79 20 74 68 65 20 6e 65 65 64 20 74 |ed.by the need t| 00000440 6f 20 72 65 66 72 65 73 68 20 74 68 65 20 64 79 |o refresh the dy| 00000450 6e 61 6d 69 63 20 52 41 4d 20 69 6e 20 74 68 65 |namic RAM in the| 00000460 20 6d 69 63 72 6f 2e 0d 0d 54 6f 20 72 65 6c 61 | micro...To rela| 00000470 74 65 20 61 20 70 69 78 65 6c 20 74 6f 20 69 74 |te a pixel to it| 00000480 73 20 73 63 72 65 65 6e 20 61 64 64 72 65 73 73 |s screen address| 00000490 20 79 6f 75 20 6e 65 65 64 20 61 20 73 69 6d 70 | you need a simp| 000004a0 6c 65 0d 66 6f 72 6d 75 6c 61 2c 20 62 75 74 20 |le.formula, but | 000004b0 6f 6e 65 20 77 68 69 63 68 20 69 6e 63 6c 75 64 |one which includ| 000004c0 65 73 20 6d 75 63 68 20 6d 75 6c 74 69 70 6c 79 |es much multiply| 000004d0 69 6e 67 20 62 79 20 26 32 38 30 2e 20 0d 53 6f |ing by &280. .So| 000004e0 20 77 72 69 74 69 6e 67 20 61 20 70 72 6f 67 72 | writing a progr| 000004f0 61 6d 20 74 6f 20 70 6f 6b 65 20 74 68 65 20 73 |am to poke the s| 00000500 63 72 65 65 6e 20 69 73 20 6e 6f 74 20 76 65 72 |creen is not ver| 00000510 79 0d 73 74 72 61 69 67 68 74 66 6f 72 77 61 72 |y.straightforwar| 00000520 64 2e 0d 0d 4d 6f 72 65 20 69 6d 70 6f 72 74 61 |d...More importa| 00000530 6e 74 6c 79 2c 20 61 6e 64 20 74 68 69 73 20 6d |ntly, and this m| 00000540 69 67 68 74 20 73 6f 75 6e 64 20 61 20 6c 69 74 |ight sound a lit| 00000550 74 6c 65 20 73 69 6c 6c 79 2c 20 62 75 74 0d 79 |tle silly, but.y| 00000560 6f 75 20 63 61 6e 20 6e 6f 74 20 62 65 20 73 75 |ou can not be su| 00000570 72 65 20 77 68 65 72 65 20 74 68 65 20 73 63 72 |re where the scr| 00000580 65 65 6e 20 77 69 6c 6c 20 62 65 20 69 6e 20 66 |een will be in f| 00000590 75 74 75 72 65 0d 6d 61 63 68 69 6e 65 73 2e 20 |uture.machines. | 000005a0 20 46 75 74 75 72 65 2d 70 72 6f 6f 66 69 6e 67 | Future-proofing| 000005b0 20 79 6f 75 72 20 6d 61 63 68 69 6e 65 20 63 6f | your machine co| 000005c0 64 65 20 69 73 20 74 68 65 0d 73 74 72 6f 6e 67 |de is the.strong| 000005d0 65 73 74 20 61 72 67 75 6d 65 6e 74 20 66 6f 72 |est argument for| 000005e0 20 6c 65 67 61 6c 20 63 6f 64 65 20 61 6e 64 2c | legal code and,| 000005f0 20 69 6e 20 61 6c 6c 20 63 61 73 65 73 20 65 78 | in all cases ex| 00000600 63 65 70 74 0d 61 72 63 61 64 65 20 67 61 6d 65 |cept.arcade game| 00000610 73 2c 20 69 73 20 70 72 65 74 74 79 20 75 6e 63 |s, is pretty unc| 00000620 68 61 6c 6c 65 6e 67 65 61 62 6c 65 2e 0d 0d 46 |hallengeable...F| 00000630 6f 72 20 65 78 61 6d 70 6c 65 2c 20 69 6e 20 61 |or example, in a| 00000640 20 42 42 43 20 4d 6f 64 65 6c 20 41 20 74 68 65 | BBC Model A the| 00000650 20 73 63 72 65 65 6e 20 61 64 64 72 65 73 73 65 | screen addresse| 00000660 73 20 61 72 65 20 26 34 30 30 30 0d 6c 6f 77 65 |s are &4000.lowe| 00000670 72 20 28 77 68 65 72 65 20 74 68 65 20 6d 6f 64 |r (where the mod| 00000680 65 73 20 65 78 69 73 74 29 20 74 68 61 6e 20 6f |es exist) than o| 00000690 6e 20 61 20 4d 6f 64 65 6c 20 42 2e 20 20 57 68 |n a Model B. Wh| 000006a0 65 6e 20 79 6f 75 0d 69 6e 74 72 6f 64 75 63 65 |en you.introduce| 000006b0 20 73 68 61 64 6f 77 20 52 41 4d 20 69 6e 20 61 | shadow RAM in a| 000006c0 20 42 2b 20 6f 72 20 61 20 4d 61 73 74 65 72 20 | B+ or a Master | 000006d0 79 6f 75 20 66 69 6e 64 20 74 68 61 74 20 79 6f |you find that yo| 000006e0 75 0d 6d 69 67 68 74 20 77 65 6c 6c 20 62 65 20 |u.might well be | 000006f0 73 75 72 65 20 6f 66 20 74 68 65 20 61 64 64 72 |sure of the addr| 00000700 65 73 73 20 62 75 74 20 79 6f 75 20 77 69 6c 6c |ess but you will| 00000710 20 6e 6f 74 20 6b 6e 6f 77 0d 77 68 65 74 68 65 | not know.whethe| 00000720 72 20 69 74 20 6c 69 65 73 20 69 6e 20 74 68 65 |r it lies in the| 00000730 20 73 61 6d 65 20 62 61 6e 6b 20 6f 66 20 52 41 | same bank of RA| 00000740 4d 20 69 6e 20 77 68 69 63 68 20 79 6f 75 72 20 |M in which your | 00000750 63 6f 64 65 0d 69 73 20 62 65 69 6e 67 20 65 78 |code.is being ex| 00000760 65 63 75 74 65 64 2e 20 20 54 68 69 73 20 61 6c |ecuted. This al| 00000770 73 6f 20 6d 65 61 6e 73 20 74 68 61 74 20 79 6f |so means that yo| 00000780 75 20 63 61 6e 20 6e 6f 74 0d 64 69 72 65 63 74 |u can not.direct| 00000790 6c 79 20 70 6f 6b 65 20 74 68 65 20 73 63 72 65 |ly poke the scre| 000007a0 65 6e 20 66 72 6f 6d 20 61 20 73 65 63 6f 6e 64 |en from a second| 000007b0 20 70 72 6f 63 65 73 73 6f 72 20 77 69 74 68 6f | processor witho| 000007c0 75 74 0d 75 73 69 6e 67 20 61 6e 20 4f 53 20 63 |ut.using an OS c| 000007d0 61 6c 6c 20 74 6f 20 63 72 6f 73 73 20 74 68 65 |all to cross the| 000007e0 20 54 75 62 65 2e 20 20 46 75 74 75 72 65 20 33 | Tube. Future 3| 000007f0 32 20 62 69 74 20 42 42 43 0d 4d 69 63 72 6f 73 |2 bit BBC.Micros| 00000800 20 6d 61 79 20 77 65 6c 6c 20 68 61 76 65 20 74 | may well have t| 00000810 68 65 20 73 63 72 65 65 6e 20 73 6f 6d 65 77 68 |he screen somewh| 00000820 65 72 65 20 74 6f 74 61 6c 6c 79 20 6e 65 77 20 |ere totally new | 00000830 61 67 61 69 6e 2e 0d 0d 54 68 65 20 74 68 69 72 |again...The thir| 00000840 64 20 64 69 73 61 64 76 61 6e 74 61 67 65 20 74 |d disadvantage t| 00000850 6f 20 64 69 72 65 63 74 6c 79 20 61 64 64 72 65 |o directly addre| 00000860 73 73 69 6e 67 20 74 68 65 20 73 63 72 65 65 6e |ssing the screen| 00000870 20 69 73 0d 74 68 61 74 20 63 68 61 6e 67 69 6e | is.that changin| 00000880 67 20 79 6f 75 72 20 73 63 72 65 65 6e 20 6d 6f |g your screen mo| 00000890 64 65 20 6d 65 61 6e 73 20 79 6f 75 20 68 61 76 |de means you hav| 000008a0 65 20 74 6f 20 63 6f 6d 70 6c 65 74 65 6c 79 0d |e to completely.| 000008b0 72 65 77 72 69 74 65 20 79 6f 75 72 20 73 63 72 |rewrite your scr| 000008c0 65 65 6e 20 61 63 63 65 73 73 69 6e 67 20 63 6f |een accessing co| 000008d0 64 65 20 73 69 6e 63 65 20 74 68 65 20 72 65 6c |de since the rel| 000008e0 61 74 69 6f 6e 20 73 68 69 70 0d 62 65 74 77 65 |ation ship.betwe| 000008f0 65 6e 20 62 79 74 65 73 20 61 6e 64 20 70 69 78 |en bytes and pix| 00000900 65 6c 73 20 69 73 20 64 69 66 66 65 72 65 6e 74 |els is different| 00000910 20 66 6f 72 20 65 61 63 68 20 6d 6f 64 65 2e 0d | for each mode..| 00000920 0d 41 73 20 61 20 67 65 6e 65 72 61 6c 20 72 75 |.As a general ru| 00000930 6c 65 20 79 6f 75 20 73 68 6f 75 6c 64 20 6e 6f |le you should no| 00000940 74 20 70 6f 6b 65 20 74 68 65 20 73 63 72 65 65 |t poke the scree| 00000950 6e 2c 20 6f 72 20 69 6e 64 65 65 64 0d 61 6e 79 |n, or indeed.any| 00000960 20 6f 74 68 65 72 20 70 61 72 74 20 6f 66 20 74 | other part of t| 00000970 68 65 20 4f 53 20 6f 72 20 42 41 53 49 43 20 77 |he OS or BASIC w| 00000980 6f 72 6b 73 70 61 63 65 20 6d 65 6d 6f 72 79 2e |orkspace memory.| 00000990 20 20 42 75 74 20 69 66 0d 79 6f 75 72 20 63 6f | But if.your co| 000009a0 64 65 20 63 6f 75 6c 64 20 62 65 20 63 6f 6e 73 |de could be cons| 000009b0 69 64 65 72 65 64 20 74 6f 20 62 65 20 61 6e 20 |idered to be an | 000009c0 65 78 74 65 6e 73 69 6f 6e 20 6f 66 20 74 68 65 |extension of the| 000009d0 20 4f 53 0d 28 73 75 63 68 20 61 73 20 61 20 6d | OS.(such as a m| 000009e0 6f 75 73 65 20 70 6f 69 6e 74 65 72 20 69 6d 70 |ouse pointer imp| 000009f0 6c 65 6d 65 6e 74 61 74 69 6f 6e 29 20 79 6f 75 |lementation) you| 00000a00 20 63 6f 75 6c 64 20 63 6c 61 69 6d 20 74 6f 0d | could claim to.| 00000a10 68 61 76 65 20 74 68 65 20 72 69 67 68 74 20 74 |have the right t| 00000a20 6f 20 64 6f 20 73 6f 2e 20 20 49 6e 20 74 68 69 |o do so. In thi| 00000a30 73 20 63 61 73 65 20 79 6f 75 20 77 6f 75 6c 64 |s case you would| 00000a40 20 68 61 76 65 20 74 6f 0d 70 72 6f 64 75 63 65 | have to.produce| 00000a50 20 61 20 64 69 66 66 65 72 65 6e 74 20 76 65 72 | a different ver| 00000a60 73 69 6f 6e 20 6f 66 20 79 6f 75 72 20 63 6f 64 |sion of your cod| 00000a70 65 20 66 6f 72 20 65 61 63 68 20 70 6f 73 73 69 |e for each possi| 00000a80 62 6c 65 0d 6d 61 63 68 69 6e 65 20 75 73 65 64 |ble.machine used| 00000a90 20 62 79 20 79 6f 75 72 20 75 73 65 72 73 2e 20 | by your users. | 00000aa0 20 49 66 20 79 6f 75 20 61 72 65 20 6a 75 73 74 | If you are just| 00000ab0 20 77 72 69 74 69 6e 67 20 66 6f 72 0d 79 6f 75 | writing for.you| 00000ac0 72 73 65 6c 66 20 74 68 65 6e 20 61 6c 6c 20 74 |rself then all t| 00000ad0 68 65 73 65 20 61 72 67 75 6d 65 6e 74 73 20 61 |hese arguments a| 00000ae0 72 65 20 73 70 75 72 69 6f 75 73 20 61 6e 79 77 |re spurious anyw| 00000af0 61 79 2e 0d 0d 4f 6e 65 20 69 6e 74 65 72 65 73 |ay...One interes| 00000b00 74 69 6e 67 6c 79 20 62 6c 75 72 72 65 64 20 61 |tingly blurred a| 00000b10 72 65 61 20 6f 66 20 69 6c 6c 65 67 61 6c 69 74 |rea of illegalit| 00000b20 79 20 28 61 73 20 64 65 66 69 6e 65 64 20 62 79 |y (as defined by| 00000b30 0d 41 63 6f 72 6e 29 20 69 73 20 56 44 55 20 76 |.Acorn) is VDU v| 00000b40 61 72 69 61 62 6c 65 73 2e 20 20 54 68 65 20 4d |ariables. The M| 00000b50 61 73 74 65 72 20 52 65 66 65 72 65 6e 63 65 20 |aster Reference | 00000b60 4d 61 6e 75 61 6c 0d 28 70 75 62 6c 69 73 68 65 |Manual.(publishe| 00000b70 64 20 62 79 20 41 63 6f 72 6e 29 20 61 63 74 75 |d by Acorn) actu| 00000b80 61 6c 6c 79 20 67 69 76 65 73 20 79 6f 75 20 69 |ally gives you i| 00000b90 6e 66 6f 72 6d 61 74 69 6f 6e 20 74 6f 0d 65 6e |nformation to.en| 00000ba0 61 62 6c 65 20 79 6f 75 20 74 6f 20 61 63 63 65 |able you to acce| 00000bb0 73 73 20 74 68 65 20 56 44 55 20 76 61 72 69 61 |ss the VDU varia| 00000bc0 62 6c 65 73 20 64 69 72 65 63 74 6c 79 2e 20 20 |bles directly. | 00000bd0 54 68 65 73 65 20 61 72 65 0d 74 68 65 20 62 79 |These are.the by| 00000be0 74 65 73 20 69 6e 20 6d 65 6d 6f 72 79 20 77 68 |tes in memory wh| 00000bf0 69 63 68 20 63 6f 6e 74 72 6f 6c 20 6d 6f 64 65 |ich control mode| 00000c00 20 64 65 66 69 6e 69 74 69 6f 6e 2c 20 61 6d 6f | definition, amo| 00000c10 6e 67 73 74 0d 6f 74 68 65 72 20 74 68 69 6e 67 |ngst.other thing| 00000c20 73 2e 20 20 48 6f 77 65 76 65 72 20 79 6f 75 20 |s. However you | 00000c30 61 72 65 20 74 6f 6c 64 20 74 6f 20 75 73 65 20 |are told to use | 00000c40 4f 53 42 59 54 45 20 63 61 6c 6c 73 20 74 6f 0d |OSBYTE calls to.| 00000c50 72 65 61 64 20 74 68 65 20 76 61 72 69 61 62 6c |read the variabl| 00000c60 65 73 20 61 6e 64 20 74 6f 20 72 65 61 64 20 74 |es and to read t| 00000c70 68 65 20 6d 65 6d 6f 72 79 20 61 64 64 72 65 73 |he memory addres| 00000c80 73 20 77 68 65 72 65 20 74 68 65 79 0d 73 74 61 |s where they.sta| 00000c90 72 74 20 61 6e 64 20 74 68 65 6e 20 6f 66 66 73 |rt and then offs| 00000ca0 65 74 20 79 6f 75 72 20 77 72 69 74 65 73 20 66 |et your writes f| 00000cb0 72 6f 6d 20 74 68 61 74 2e 20 20 49 66 2c 20 69 |rom that. If, i| 00000cc0 6e 20 66 75 74 75 72 65 0d 6d 61 63 68 69 6e 65 |n future.machine| 00000cd0 73 2c 20 74 68 65 20 74 61 62 6c 65 20 6d 6f 76 |s, the table mov| 00000ce0 65 73 20 74 68 65 6e 20 79 6f 75 72 20 63 6f 64 |es then your cod| 00000cf0 65 20 73 68 6f 75 6c 64 20 73 74 69 6c 6c 20 77 |e should still w| 00000d00 6f 72 6b 0d 69 66 20 79 6f 75 20 75 73 65 64 20 |ork.if you used | 00000d10 74 68 65 20 72 65 6c 65 76 61 6e 74 20 4f 53 42 |the relevant OSB| 00000d20 59 54 45 20 63 61 6c 6c 73 2e 0d 0d 54 68 65 20 |YTE calls...The | 00000d30 42 42 43 20 4d 69 63 72 6f 20 6f 70 65 72 61 74 |BBC Micro operat| 00000d40 69 6e 67 20 73 79 73 74 65 6d 20 70 72 6f 76 69 |ing system provi| 00000d50 64 65 73 20 61 20 77 69 64 65 20 72 61 6e 67 65 |des a wide range| 00000d60 20 6f 66 0d 63 61 6c 6c 73 20 74 6f 20 68 65 6c | of.calls to hel| 00000d70 70 20 79 6f 75 72 20 70 72 6f 67 72 61 6d 6d 69 |p your programmi| 00000d80 6e 67 2e 20 20 41 63 6f 72 6e 27 73 20 6d 69 63 |ng. Acorn's mic| 00000d90 72 6f 73 20 64 6f 20 6e 6f 74 20 68 61 76 65 0d |ros do not have.| 00000da0 61 20 63 6c 6f 73 65 64 20 61 6e 64 20 75 6e 64 |a closed and und| 00000db0 6f 63 75 6d 65 6e 74 65 64 20 4f 53 20 6c 69 6b |ocumented OS lik| 00000dc0 65 20 73 6f 6d 65 20 68 6f 6d 65 20 6d 61 63 68 |e some home mach| 00000dd0 69 6e 65 73 2c 20 61 6e 64 0d 65 76 65 6e 20 73 |ines, and.even s| 00000de0 6f 6d 65 20 6d 61 69 6e 66 72 61 6d 65 73 2e 20 |ome mainframes. | 00000df0 20 53 6f 2c 20 69 66 20 79 6f 75 20 63 61 6e 2c | So, if you can,| 00000e00 20 75 73 65 20 74 68 65 20 72 65 6c 65 76 61 6e | use the relevan| 00000e10 74 20 4f 53 0d 63 61 6c 6c 2e 0d 0d 4f 6e 65 20 |t OS.call...One | 00000e20 6f 66 20 74 68 65 20 74 6f 70 20 70 72 6f 67 72 |of the top progr| 00000e30 61 6d 6d 65 72 73 20 61 74 20 41 63 6f 72 6e 2c |ammers at Acorn,| 00000e40 20 61 6e 64 20 74 68 65 20 6d 61 6e 20 77 68 6f | and the man who| 00000e50 20 77 72 6f 74 65 0d 42 42 43 20 42 41 53 49 43 | wrote.BBC BASIC| 00000e60 2c 20 6f 66 74 65 6e 20 73 61 79 73 20 74 68 61 |, often says tha| 00000e70 74 20 74 68 65 20 62 65 73 74 20 74 65 73 74 20 |t the best test | 00000e80 6f 66 20 6c 65 67 61 6c 69 74 79 20 69 73 20 69 |of legality is i| 00000e90 66 0d 74 68 65 20 63 6f 64 65 20 77 69 6c 6c 20 |f.the code will | 00000ea0 72 75 6e 20 69 6e 20 61 20 36 35 30 32 20 73 65 |run in a 6502 se| 00000eb0 63 6f 6e 64 20 70 72 6f 63 65 73 73 6f 72 20 61 |cond processor a| 00000ec0 6e 64 20 64 6f 20 69 74 73 20 6a 6f 62 0d 63 6f |nd do its job.co| 00000ed0 72 72 65 63 74 6c 79 2e 20 20 41 6c 74 68 6f 75 |rrectly. Althou| 00000ee0 67 68 20 74 68 65 72 65 20 61 72 65 20 73 6f 6d |gh there are som| 00000ef0 65 20 6c 65 67 61 6c 20 74 72 69 63 6b 73 20 74 |e legal tricks t| 00000f00 68 61 74 20 77 6f 72 6b 0d 6f 6e 6c 79 20 69 6e |hat work.only in| 00000f10 20 74 68 65 20 6d 61 69 6e 20 6d 69 63 72 6f 20 | the main micro | 00000f20 74 68 69 73 20 69 73 20 61 20 67 6f 6f 64 20 72 |this is a good r| 00000f30 75 6c 65 20 6f 66 20 74 68 75 6d 62 2e 0d 0d 54 |ule of thumb...T| 00000f40 68 65 20 70 72 6f 67 72 61 6d 20 69 6e 20 74 68 |he program in th| 00000f50 69 73 20 6d 6f 64 75 6c 65 20 69 73 20 61 20 73 |is module is a s| 00000f60 63 72 65 65 6e 20 66 69 6c 6c 20 72 6f 75 74 69 |creen fill routi| 00000f70 6e 65 20 74 68 61 74 0d 61 64 64 72 65 73 73 65 |ne that.addresse| 00000f80 73 20 74 68 65 20 73 63 72 65 65 6e 20 64 69 72 |s the screen dir| 00000f90 65 63 74 6c 79 2e 20 20 54 6f 20 75 73 65 20 69 |ectly. 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OS\BITS/T\OSB25.m0
OS\BITS/T\OSB25.m1
OS\BITS/T\OSB25.m2
OS\BITS/T\OSB25.m4
OS\BITS/T\OSB25.m5
.