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V/+BRR1

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 » Personal collection » Acorn ADFS disks » Greaseweazled » adfs_EUG_55.adf
Filename: V/+BRR1
Read OK: ✔
File size: 147F bytes
Load address: 56204556
Exec address: 52422B2E
Duplicates

There are 3 duplicate copies of this file in the archive:

File contents
                          BASIC ROM ROUTINES
                         Christopher Dewhurst

THE Basic Rom is that dark and mysterious area of memory that lies 
beyond the screen, starting at &8000 and stretching up to &BFFF. It's
the backstage department containing the machine code needed to interpret
your Basic commands. But have you ever wondered if we can use some of 
that machine code in our own programs? Have you, for instance, struggled
to write an assembler routine to print a number on the screen, when one
must surely exist somewhere in the Basic Rom? Well, wonder no more, 
because there is indeed such a routine, and in this article we'll be 
exploring that and a lot more besides.
   Before we go any further, however, a word of caution. The best 
machine code is specific machine code written for a specific job; Basic
Rom routines are general-purpose routines, and are not the answer to
everything. Having said that, if speed is not your main priority, then
the Rom routines are ideal. They make your programs smaller and smarter,
provided you use them properly - and this usually involves some fairly
tricky setting up - so listen carefully.
   I learnt a lot about the Basic Rom by exploring around it and experi-
menting with it myself. I also picked up a few tips from THE ADVANCED
BASIC ROM USER GUIDE by Colin Pharo (Cambridge Micro Centre, 1984). 
Roland Waddilove also presented a series of excellent articles on the 
subject in ELECTRON USER; if you still have these paper beauties, dig 
out the November 1988 issue for a rundown on mathematical Rom routines. 
However, I will be concentrating on routines which print numbers in hex 
or decimal, the random number generator, and printing strings of text.
   In case you're wondering, BBC Master owners won't be left out of the
discussion this time. I have done quite a bit of disassembling of the
Basic 4 Rom to find out where equivalent routines to Basic 2 reside. 
Basic 2 is the Rom fitted in the BBC B and Electron, and Basic 4 is the 
one fitted in the Master (Like the Plus 2, for some reason Basic 3
never was). When I talk about a Rom routine, I will specify both the
Basic 2 and Basic 4 addresses - together with examples and commentaries
on how to use them - so it is up to you to use the correct one depending
on which computer you have. If you experience any difficulties - or if 
you have additional hints and tips - just write in.
   Right, down to business. We must first get to know what is called the
Integer Work Area, or IWA for short. This is just a sequence of four
bytes in zero page, located at &2A-&2D. Before Basic can work on an
integer variable, be it adding a number to it or printing it out, it 
must be put into the IWA. Fortunately, life is made easier with the help
of a couple of routines which copy an integer variable, either from zero
page or from the main memory, to the IWA:

1.      Routine: Copy 4-byte integer from zero page to the IWA
        Basic 2 address: &AF56
        Basic 4 address: &AA80

        Entry: X = zero page offset at which the integer to be copied is
        located.
        Exit: IWA contains the integer.
        Ex.:    LDX #&70  \integer at &70-3
                JSR &AF56  \copy to IWA

2.      Routine: Copy 4-byte integer from memory to the IWA
        Basic 2: &B336
        Basic 4: &B1AA

        Entry: &2A/&2B contain address of the integer.
        Exit: IWA contains the integer.
        Ex.:    LDA #integer MOD 256
                STA &2A
                LDA #integer DIV 256
                STA &2B	
                JSR &B336
                ...
                .integer EQUD &12345678

There are also two routines which do the opposite of above. The one at 
&BE44 (Basic 2)/&BDC6 (Basic 4) copies the IWA to a zero-page location, 
X being set to the zero page location on entry. The routine at &B4C6 
(Basic 2)/&B347 (Basic 4) copies the IWA to a location in main memory 
whose address is held in &37/&38.

3.      Routine: Print a string
        Basic 2: &BFCF
        Basic 4: &BECF

        Entry: The string must follow the JSR &BFCF instruction, and be
        terminated by a byte of value &80 or greater.
        Ex.:    JSR &BFCF
                EQUS "Hello there.":NOP
                ...

Notice how I've used the NOP instruction to terminate the string. The
NOP opcode has a value of &EA, which satisfies the condition of being
&80 or greater. The important point to remember is that program
execution continues AFTER that NOP instruction. In machine code, every
time a JSR instruction is executed the current address is pushed onto
the stack. Basic pulls this address from the stack, stores it in zero
page locations &37/&38 and uses indirect addressing to get the bytes of
the string. By the time the string has been printed, &37/&38 contains
the address of the next instruction after the string in the program
that called the routine. The disadvantage of this routine, however, is
that while you can include control codes (to turn off the cursor for
instance) you can't print out a string of graphics characters because
they have an ASCII value of &80 or above, which, as we said, is used to
terminate the string.

This article continued in V.+BRR2. First published EUG #55.

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00000010  20 20 20 20 20 20 20 20  20 20 42 41 53 49 43 20  |          BASIC |
00000020  52 4f 4d 20 52 4f 55 54  49 4e 45 53 0d 20 20 20  |ROM ROUTINES.   |
00000030  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
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00000af0  68 69 63 68 20 63 6f 70  79 20 61 6e 20 69 6e 74  |hich copy an int|
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00000b30  20 6d 61 69 6e 20 6d 65  6d 6f 72 79 2c 20 74 6f  | main memory, to|
00000b40  20 74 68 65 20 49 57 41  3a 0d 0d 31 2e 20 20 20  | the IWA:..1.   |
00000b50  20 20 20 52 6f 75 74 69  6e 65 3a 20 43 6f 70 79  |   Routine: Copy|
00000b60  20 34 2d 62 79 74 65 20  69 6e 74 65 67 65 72 20  | 4-byte integer |
00000b70  66 72 6f 6d 20 7a 65 72  6f 20 70 61 67 65 20 74  |from zero page t|
00000b80  6f 20 74 68 65 20 49 57  41 0d 20 20 20 20 20 20  |o the IWA.      |
00000b90  20 20 42 61 73 69 63 20  32 20 61 64 64 72 65 73  |  Basic 2 addres|
00000ba0  73 3a 20 26 41 46 35 36  0d 20 20 20 20 20 20 20  |s: &AF56.       |
00000bb0  20 42 61 73 69 63 20 34  20 61 64 64 72 65 73 73  | Basic 4 address|
00000bc0  3a 20 26 41 41 38 30 0d  0d 20 20 20 20 20 20 20  |: &AA80..       |
00000bd0  20 45 6e 74 72 79 3a 20  58 20 3d 20 7a 65 72 6f  | Entry: X = zero|
00000be0  20 70 61 67 65 20 6f 66  66 73 65 74 20 61 74 20  | page offset at |
00000bf0  77 68 69 63 68 20 74 68  65 20 69 6e 74 65 67 65  |which the intege|
00000c00  72 20 74 6f 20 62 65 20  63 6f 70 69 65 64 20 69  |r to be copied i|
00000c10  73 0d 20 20 20 20 20 20  20 20 6c 6f 63 61 74 65  |s.        locate|
00000c20  64 2e 0d 20 20 20 20 20  20 20 20 45 78 69 74 3a  |d..        Exit:|
00000c30  20 49 57 41 20 63 6f 6e  74 61 69 6e 73 20 74 68  | IWA contains th|
00000c40  65 20 69 6e 74 65 67 65  72 2e 0d 20 20 20 20 20  |e integer..     |
00000c50  20 20 20 45 78 2e 3a 20  20 20 20 4c 44 58 20 23  |   Ex.:    LDX #|
00000c60  26 37 30 20 20 5c 69 6e  74 65 67 65 72 20 61 74  |&70  \integer at|
00000c70  20 26 37 30 2d 33 0d 20  20 20 20 20 20 20 20 20  | &70-3.         |
00000c80  20 20 20 20 20 20 20 4a  53 52 20 26 41 46 35 36  |       JSR &AF56|
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00000ca0  32 2e 20 20 20 20 20 20  52 6f 75 74 69 6e 65 3a  |2.      Routine:|
00000cb0  20 43 6f 70 79 20 34 2d  62 79 74 65 20 69 6e 74  | Copy 4-byte int|
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00000cf0  33 36 0d 20 20 20 20 20  20 20 20 42 61 73 69 63  |36.        Basic|
00000d00  20 34 3a 20 26 42 31 41  41 0d 0d 20 20 20 20 20  | 4: &B1AA..     |
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00000d20  42 20 63 6f 6e 74 61 69  6e 20 61 64 64 72 65 73  |B contain addres|
00000d30  73 20 6f 66 20 74 68 65  20 69 6e 74 65 67 65 72  |s of the integer|
00000d40  2e 0d 20 20 20 20 20 20  20 20 45 78 69 74 3a 20  |..        Exit: |
00000d50  49 57 41 20 63 6f 6e 74  61 69 6e 73 20 74 68 65  |IWA contains the|
00000d60  20 69 6e 74 65 67 65 72  2e 0d 20 20 20 20 20 20  | integer..      |
00000d70  20 20 45 78 2e 3a 20 20  20 20 4c 44 41 20 23 69  |  Ex.:    LDA #i|
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00000dc0  67 65 72 20 44 49 56 20  32 35 36 0d 20 20 20 20  |ger DIV 256.    |
00000dd0  20 20 20 20 20 20 20 20  20 20 20 20 53 54 41 20  |            STA |
00000de0  26 32 42 09 0d 20 20 20  20 20 20 20 20 20 20 20  |&2B..           |
00000df0  20 20 20 20 20 4a 53 52  20 26 42 33 33 36 0d 20  |     JSR &B336. |
00000e00  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 2e  |               .|
00000e10  2e 2e 0d 20 20 20 20 20  20 20 20 20 20 20 20 20  |...             |
00000e20  20 20 20 2e 69 6e 74 65  67 65 72 20 45 51 55 44  |   .integer EQUD|
00000e30  20 26 31 32 33 34 35 36  37 38 0d 0d 54 68 65 72  | &12345678..Ther|
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00000e60  20 74 68 65 20 6f 70 70  6f 73 69 74 65 20 6f 66  | the opposite of|
00000e70  20 61 62 6f 76 65 2e 20  54 68 65 20 6f 6e 65 20  | above. The one |
00000e80  61 74 20 0d 26 42 45 34  34 20 28 42 61 73 69 63  |at .&BE44 (Basic|
00000e90  20 32 29 2f 26 42 44 43  36 20 28 42 61 73 69 63  | 2)/&BDC6 (Basic|
00000ea0  20 34 29 20 63 6f 70 69  65 73 20 74 68 65 20 49  | 4) copies the I|
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00000ed0  65 69 6e 67 20 73 65 74  20 74 6f 20 74 68 65 20  |eing set to the |
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00000ef0  6f 6e 20 6f 6e 20 65 6e  74 72 79 2e 20 54 68 65  |on on entry. The|
00000f00  20 72 6f 75 74 69 6e 65  20 61 74 20 26 42 34 43  | routine at &B4C|
00000f10  36 20 0d 28 42 61 73 69  63 20 32 29 2f 26 42 33  |6 .(Basic 2)/&B3|
00000f20  34 37 20 28 42 61 73 69  63 20 34 29 20 63 6f 70  |47 (Basic 4) cop|
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00000f40  20 6c 6f 63 61 74 69 6f  6e 20 69 6e 20 6d 61 69  | location in mai|
00000f50  6e 20 6d 65 6d 6f 72 79  20 0d 77 68 6f 73 65 20  |n memory .whose |
00000f60  61 64 64 72 65 73 73 20  69 73 20 68 65 6c 64 20  |address is held |
00000f70  69 6e 20 26 33 37 2f 26  33 38 2e 0d 0d 33 2e 20  |in &37/&38...3. |
00000f80  20 20 20 20 20 52 6f 75  74 69 6e 65 3a 20 50 72  |     Routine: Pr|
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00000fb0  46 43 46 0d 20 20 20 20  20 20 20 20 42 61 73 69  |FCF.        Basi|
00000fc0  63 20 34 3a 20 26 42 45  43 46 0d 0d 20 20 20 20  |c 4: &BECF..    |
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00000fe0  74 72 69 6e 67 20 6d 75  73 74 20 66 6f 6c 6c 6f  |tring must follo|
00000ff0  77 20 74 68 65 20 4a 53  52 20 26 42 46 43 46 20  |w the JSR &BFCF |
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00001010  20 62 65 0d 20 20 20 20  20 20 20 20 74 65 72 6d  | be.        term|
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00001040  20 67 72 65 61 74 65 72  2e 0d 20 20 20 20 20 20  | greater..      |
00001050  20 20 45 78 2e 3a 20 20  20 20 4a 53 52 20 26 42  |  Ex.:    JSR &B|
00001060  46 43 46 0d 20 20 20 20  20 20 20 20 20 20 20 20  |FCF.            |
00001070  20 20 20 20 45 51 55 53  20 22 48 65 6c 6c 6f 20  |    EQUS "Hello |
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00001090  20 20 20 20 20 20 20 20  20 20 20 20 2e 2e 2e 0d  |            ....|
000010a0  0d 4e 6f 74 69 63 65 20  68 6f 77 20 49 27 76 65  |.Notice how I've|
000010b0  20 75 73 65 64 20 74 68  65 20 4e 4f 50 20 69 6e  | used the NOP in|
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00001120  69 6f 6e 20 6f 66 20 62  65 69 6e 67 0d 26 38 30  |ion of being.&80|
00001130  20 6f 72 20 67 72 65 61  74 65 72 2e 20 54 68 65  | or greater. The|
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00001150  20 74 6f 20 72 65 6d 65  6d 62 65 72 20 69 73 20  | to remember is |
00001160  74 68 61 74 20 70 72 6f  67 72 61 6d 0d 65 78 65  |that program.exe|
00001170  63 75 74 69 6f 6e 20 63  6f 6e 74 69 6e 75 65 73  |cution continues|
00001180  20 41 46 54 45 52 20 74  68 61 74 20 4e 4f 50 20  | AFTER that NOP |
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00001200  61 63 6b 2e 20 42 61 73  69 63 20 70 75 6c 6c 73  |ack. Basic pulls|
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00001240  70 61 67 65 20 6c 6f 63  61 74 69 6f 6e 73 20 26  |page locations &|
00001250  33 37 2f 26 33 38 20 61  6e 64 20 75 73 65 73 20  |37/&38 and uses |
00001260  69 6e 64 69 72 65 63 74  20 61 64 64 72 65 73 73  |indirect address|
00001270  69 6e 67 20 74 6f 20 67  65 74 20 74 68 65 20 62  |ing to get the b|
00001280  79 74 65 73 20 6f 66 0d  74 68 65 20 73 74 72 69  |ytes of.the stri|
00001290  6e 67 2e 20 42 79 20 74  68 65 20 74 69 6d 65 20  |ng. By the time |
000012a0  74 68 65 20 73 74 72 69  6e 67 20 68 61 73 20 62  |the string has b|
000012b0  65 65 6e 20 70 72 69 6e  74 65 64 2c 20 26 33 37  |een printed, &37|
000012c0  2f 26 33 38 20 63 6f 6e  74 61 69 6e 73 0d 74 68  |/&38 contains.th|
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00001310  6d 0d 74 68 61 74 20 63  61 6c 6c 65 64 20 74 68  |m.that called th|
00001320  65 20 72 6f 75 74 69 6e  65 2e 20 54 68 65 20 64  |e routine. The d|
00001330  69 73 61 64 76 61 6e 74  61 67 65 20 6f 66 20 74  |isadvantage of t|
00001340  68 69 73 20 72 6f 75 74  69 6e 65 2c 20 68 6f 77  |his routine, how|
00001350  65 76 65 72 2c 20 69 73  0d 74 68 61 74 20 77 68  |ever, is.that wh|
00001360  69 6c 65 20 79 6f 75 20  63 61 6e 20 69 6e 63 6c  |ile you can incl|
00001370  75 64 65 20 63 6f 6e 74  72 6f 6c 20 63 6f 64 65  |ude control code|
00001380  73 20 28 74 6f 20 74 75  72 6e 20 6f 66 66 20 74  |s (to turn off t|
00001390  68 65 20 63 75 72 73 6f  72 20 66 6f 72 0d 69 6e  |he cursor for.in|
000013a0  73 74 61 6e 63 65 29 20  79 6f 75 20 63 61 6e 27  |stance) you can'|
000013b0  74 20 70 72 69 6e 74 20  6f 75 74 20 61 20 73 74  |t print out a st|
000013c0  72 69 6e 67 20 6f 66 20  67 72 61 70 68 69 63 73  |ring of graphics|
000013d0  20 63 68 61 72 61 63 74  65 72 73 20 62 65 63 61  | characters beca|
000013e0  75 73 65 0d 74 68 65 79  20 68 61 76 65 20 61 6e  |use.they have an|
000013f0  20 41 53 43 49 49 20 76  61 6c 75 65 20 6f 66 20  | ASCII value of |
00001400  26 38 30 20 6f 72 20 61  62 6f 76 65 2c 20 77 68  |&80 or above, wh|
00001410  69 63 68 2c 20 61 73 20  77 65 20 73 61 69 64 2c  |ich, as we said,|
00001420  20 69 73 20 75 73 65 64  20 74 6f 0d 74 65 72 6d  | is used to.term|
00001430  69 6e 61 74 65 20 74 68  65 20 73 74 72 69 6e 67  |inate the string|
00001440  2e 0d 0d 54 68 69 73 20  61 72 74 69 63 6c 65 20  |...This article |
00001450  63 6f 6e 74 69 6e 75 65  64 20 69 6e 20 56 2e 2b  |continued in V.+|
00001460  42 52 52 32 2e 20 46 69  72 73 74 20 70 75 62 6c  |BRR2. First publ|
00001470  69 73 68 65 64 20 45 55  47 20 23 35 35 2e 0d     |ished EUG #55..|
0000147f
V/+BRR1.m0
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