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11_12_87/T\OSB08
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 » telesoftware3.adl |
| Filename: | 11_12_87/T\OSB08 |
| Read OK: | ✔ |
| File size: | 123A bytes |
| Load address: | 0000 |
| Exec address: | 0000 |
File contents
OSBITS - An Exploration of the BBC Micro at Machine Level
By Programmer
...........................................................
Part 8: Logical Operators
We're back to comparisons in this module. Already you have
come across branching operations carried out conditionally
on the setting of certain flags in the processor status
register; mnemonics like BNE BEQ BMI. Logical operators
enable us to compare the bits of one byte with the bits of
another, and essentially there are three types of
comparison.
AND is the first. This means that when you take byte1 AND
byte 2 the result has bits set only where they are set in
both bytes. This can be illustrated by using a thing called
a truth table (ancient form of torture no doubt):
0 1 0 AND 0 = 0
1 AND 0 = 0
0 0 0 0 AND 1 = 0
1 0 1 1 AND 1 = 1
AND is used to mask out certain bits of a byte so that we
are only left with the ones we want. Say we wanted to set
the top bit of a byte held in the Accumulator to zero, then
we would do it like this.
LDA number
AND #&7F \ &7F is 01111111 in binary
This does not mean that all seven lower bits of the number
are set but that the top bit is cleared and the lower seven
are left unchanged.
In the program for the last module I used AND #&F to mask
out the top four bits of a byte so I could work with the
just the bottom nybble.
There is a variant on AND with the mnemonic BIT which
performs an AND without changing the byte in the
Accumulator. It does affect the flags though so branching
can be done on the result of a BIT operation. It is used to
test specific bits (hence the mnemonic) and to use it you
load the mask into the accumulator and BIT a memory location
like this:
LDA #1
BIT number \ If bit 0 is 1 then the zero
\ flag will be cleared
BNE elsewhere \ So branch if bit 0 is set
OR is the next, with the mnemonic ORA. Here if a particular
bit of either byte is set then that bit of the result will
be set.
0 1 0 0R 0 = 0
1 OR 0 = 1
0 0 1 0 OR 1 = 1
1 1 1 1 OR 1 = 1
This is used to make sure that bits of a byte are set (i.e.
the reverse of AND) such that to set the top bit of a number
we do this:
LDA number
ORA &80 \ &80 is 10000000 in binary
and the top bit will be set and the rest will remain
unchanged.
The final operator is the Exclusive OR, and EOR is the
mnemonic. Here a bit in the result is set if that bit is
set in either, but not both, of the other bytes.
0 1 0 EOR 0 = 0
1 EOR 0 = 1
0 0 1 0 EOR 1 = 1
1 1 0 1 EOR 1 = 0
With this you can reverse the status of a flag without
knowing that status beforehand. I call it flag waving with
EOR (apologies to A A Milne). Let's say you have bit 0 of a
number as a flag and you want to reverse it, then you do
this.
LDA number
EOR 1 \ 1 is, of course, 00000001
EORing a number with itself always produces zero and EORing
with &FF reverses the settings of all bits, producing a ones
complement negative. A neat way to produce the negative of
a byte is to EOR it with &FF (255) and then add one. Check
back to module 2 to see why it works.
It's a simple little piece of code in this module. The
ASCII codes are arranged such that the code for a letter AND
&DF will always produce the code for upper case, and B/osb08
does simply that. However, being simple it does not produce
a satisfactory result for all the other keys on the
keyboard. This approach is one of the ways to legislate
against people who turn off (or on) the CAPS LOCK on their
micro when you expect it the other way around. The other
way is to use OSBYTE 202 which simulates pressing of the
relevant keys. The only trap in this code is to prevent a
control code sneaking through without you expecting it.
In the next module I'll give you some information about the
other operating system routines in the BBC Micro to do with
file handling and the like, and we'll use one of them to
print out a list of the ROMS in your machine.
............................................................
PS: Thank you for your feedback. I'm particularly grateful
to Peter Vince for his constructive criticism and spelling
checker and I hope that my odd typo and (rare!) brain clog
have not confused you.
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 2e 2e 0d 0d 0d 50 61 72 74 20 38 3a 20 |........Part 8: | 00000090 4c 6f 67 69 63 61 6c 20 4f 70 65 72 61 74 6f 72 |Logical Operator| 000000a0 73 0d 0d 57 65 27 72 65 20 62 61 63 6b 20 74 6f |s..We're back to| 000000b0 20 63 6f 6d 70 61 72 69 73 6f 6e 73 20 69 6e 20 | comparisons in | 000000c0 74 68 69 73 20 6d 6f 64 75 6c 65 2e 20 20 41 6c |this module. Al| 000000d0 72 65 61 64 79 20 79 6f 75 20 68 61 76 65 0d 63 |ready you have.c| 000000e0 6f 6d 65 20 61 63 72 6f 73 73 20 62 72 61 6e 63 |ome across branc| 000000f0 68 69 6e 67 20 6f 70 65 72 61 74 69 6f 6e 73 20 |hing operations | 00000100 63 61 72 72 69 65 64 20 6f 75 74 20 63 6f 6e 64 |carried out cond| 00000110 69 74 69 6f 6e 61 6c 6c 79 0d 6f 6e 20 74 68 65 |itionally.on the| 00000120 20 73 65 74 74 69 6e 67 20 6f 66 20 63 65 72 74 | setting of cert| 00000130 61 69 6e 20 66 6c 61 67 73 20 69 6e 20 74 68 65 |ain flags in the| 00000140 20 70 72 6f 63 65 73 73 6f 72 20 73 74 61 74 75 | processor statu| 00000150 73 0d 72 65 67 69 73 74 65 72 3b 20 6d 6e 65 6d |s.register; mnem| 00000160 6f 6e 69 63 73 20 6c 69 6b 65 20 42 4e 45 20 42 |onics like BNE B| 00000170 45 51 20 42 4d 49 2e 20 20 4c 6f 67 69 63 61 6c |EQ BMI. Logical| 00000180 20 6f 70 65 72 61 74 6f 72 73 0d 65 6e 61 62 6c | operators.enabl| 00000190 65 20 75 73 20 74 6f 20 63 6f 6d 70 61 72 65 20 |e us to compare | 000001a0 74 68 65 20 62 69 74 73 20 6f 66 20 6f 6e 65 20 |the bits of one | 000001b0 62 79 74 65 20 77 69 74 68 20 74 68 65 20 62 69 |byte with the bi| 000001c0 74 73 20 6f 66 0d 61 6e 6f 74 68 65 72 2c 20 61 |ts of.another, a| 000001d0 6e 64 20 65 73 73 65 6e 74 69 61 6c 6c 79 20 74 |nd essentially t| 000001e0 68 65 72 65 20 61 72 65 20 74 68 72 65 65 20 74 |here are three t| 000001f0 79 70 65 73 20 6f 66 0d 63 6f 6d 70 61 72 69 73 |ypes of.comparis| 00000200 6f 6e 2e 0d 0d 41 4e 44 20 69 73 20 74 68 65 20 |on...AND is the | 00000210 66 69 72 73 74 2e 20 20 54 68 69 73 20 6d 65 61 |first. This mea| 00000220 6e 73 20 74 68 61 74 20 77 68 65 6e 20 79 6f 75 |ns that when you| 00000230 20 74 61 6b 65 20 62 79 74 65 31 20 41 4e 44 0d | take byte1 AND.| 00000240 62 79 74 65 20 32 20 74 68 65 20 72 65 73 75 6c |byte 2 the resul| 00000250 74 20 68 61 73 20 62 69 74 73 20 73 65 74 20 6f |t has bits set o| 00000260 6e 6c 79 20 77 68 65 72 65 20 74 68 65 79 20 61 |nly where they a| 00000270 72 65 20 73 65 74 20 69 6e 0d 62 6f 74 68 20 62 |re set in.both b| 00000280 79 74 65 73 2e 20 20 54 68 69 73 20 63 61 6e 20 |ytes. This can | 00000290 62 65 20 69 6c 6c 75 73 74 72 61 74 65 64 20 62 |be illustrated b| 000002a0 79 20 75 73 69 6e 67 20 61 20 74 68 69 6e 67 20 |y using a thing | 000002b0 63 61 6c 6c 65 64 0d 61 20 74 72 75 74 68 20 74 |called.a truth t| 000002c0 61 62 6c 65 20 28 61 6e 63 69 65 6e 74 20 66 6f |able (ancient fo| 000002d0 72 6d 20 6f 66 20 74 6f 72 74 75 72 65 20 6e 6f |rm of torture no| 000002e0 20 64 6f 75 62 74 29 3a 0d 0d 20 20 20 20 20 20 | doubt):.. | 000002f0 20 20 20 20 20 20 20 20 20 20 20 20 20 20 30 20 | 0 | 00000300 20 31 20 20 20 20 20 20 20 20 20 20 30 20 41 4e | 1 0 AN| 00000310 44 20 30 20 3d 20 30 0d 20 20 20 20 20 20 20 20 |D 0 = 0. | 00000320 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000330 20 20 20 20 20 20 20 20 20 20 31 20 41 4e 44 20 | 1 AND | 00000340 30 20 3d 20 30 0d 20 20 20 20 20 20 20 20 20 20 |0 = 0. | 00000350 20 20 20 20 20 30 20 20 20 20 30 20 20 30 20 20 | 0 0 0 | 00000360 20 20 20 20 20 20 20 20 30 20 41 4e 44 20 31 20 | 0 AND 1 | 00000370 3d 20 30 0d 20 20 20 20 20 20 20 20 20 20 20 20 |= 0. | 00000380 20 20 20 31 20 20 20 20 30 20 20 31 20 20 20 20 | 1 0 1 | 00000390 20 20 20 20 20 20 31 20 41 4e 44 20 31 20 3d 20 | 1 AND 1 = | 000003a0 31 0d 0d 41 4e 44 20 69 73 20 75 73 65 64 20 74 |1..AND is used t| 000003b0 6f 20 6d 61 73 6b 20 6f 75 74 20 63 65 72 74 61 |o mask out certa| 000003c0 69 6e 20 62 69 74 73 20 6f 66 20 61 20 62 79 74 |in bits of a byt| 000003d0 65 20 73 6f 20 74 68 61 74 20 77 65 0d 61 72 65 |e so that we.are| 000003e0 20 6f 6e 6c 79 20 6c 65 66 74 20 77 69 74 68 20 | only left with | 000003f0 74 68 65 20 6f 6e 65 73 20 77 65 20 77 61 6e 74 |the ones we want| 00000400 2e 20 20 53 61 79 20 77 65 20 77 61 6e 74 65 64 |. Say we wanted| 00000410 20 74 6f 20 73 65 74 0d 74 68 65 20 74 6f 70 20 | to set.the top | 00000420 62 69 74 20 6f 66 20 61 20 62 79 74 65 20 68 65 |bit of a byte he| 00000430 6c 64 20 69 6e 20 74 68 65 20 41 63 63 75 6d 75 |ld in the Accumu| 00000440 6c 61 74 6f 72 20 74 6f 20 7a 65 72 6f 2c 20 74 |lator to zero, t| 00000450 68 65 6e 0d 77 65 20 77 6f 75 6c 64 20 64 6f 20 |hen.we would do | 00000460 69 74 20 6c 69 6b 65 20 74 68 69 73 2e 0d 0d 20 |it like this... | 00000470 20 20 20 20 20 20 20 4c 44 41 20 6e 75 6d 62 65 | LDA numbe| 00000480 72 0d 20 20 20 20 20 20 20 20 41 4e 44 20 23 26 |r. AND #&| 00000490 37 46 20 20 20 20 20 20 20 20 20 5c 20 26 37 46 |7F \ &7F| 000004a0 20 69 73 20 30 31 31 31 31 31 31 31 20 69 6e 20 | is 01111111 in | 000004b0 62 69 6e 61 72 79 0d 0d 54 68 69 73 20 64 6f 65 |binary..This doe| 000004c0 73 20 6e 6f 74 20 6d 65 61 6e 20 74 68 61 74 20 |s not mean that | 000004d0 61 6c 6c 20 73 65 76 65 6e 20 6c 6f 77 65 72 20 |all seven lower | 000004e0 62 69 74 73 20 6f 66 20 74 68 65 20 6e 75 6d 62 |bits of the numb| 000004f0 65 72 0d 61 72 65 20 73 65 74 20 62 75 74 20 74 |er.are set but t| 00000500 68 61 74 20 74 68 65 20 74 6f 70 20 62 69 74 20 |hat the top bit | 00000510 69 73 20 63 6c 65 61 72 65 64 20 61 6e 64 20 74 |is cleared and t| 00000520 68 65 20 6c 6f 77 65 72 20 73 65 76 65 6e 0d 61 |he lower seven.a| 00000530 72 65 20 6c 65 66 74 20 75 6e 63 68 61 6e 67 65 |re left unchange| 00000540 64 2e 0d 0d 49 6e 20 74 68 65 20 70 72 6f 67 72 |d...In the progr| 00000550 61 6d 20 66 6f 72 20 74 68 65 20 6c 61 73 74 20 |am for the last | 00000560 6d 6f 64 75 6c 65 20 49 20 75 73 65 64 20 41 4e |module I used AN| 00000570 44 20 23 26 46 20 74 6f 20 6d 61 73 6b 0d 6f 75 |D #&F to mask.ou| 00000580 74 20 74 68 65 20 74 6f 70 20 66 6f 75 72 20 62 |t the top four b| 00000590 69 74 73 20 6f 66 20 61 20 62 79 74 65 20 73 6f |its of a byte so| 000005a0 20 49 20 63 6f 75 6c 64 20 77 6f 72 6b 20 77 69 | I could work wi| 000005b0 74 68 20 74 68 65 0d 6a 75 73 74 20 74 68 65 20 |th the.just the | 000005c0 62 6f 74 74 6f 6d 20 6e 79 62 62 6c 65 2e 0d 0d |bottom nybble...| 000005d0 54 68 65 72 65 20 69 73 20 61 20 76 61 72 69 61 |There is a varia| 000005e0 6e 74 20 6f 6e 20 41 4e 44 20 77 69 74 68 20 74 |nt on AND with t| 000005f0 68 65 20 6d 6e 65 6d 6f 6e 69 63 20 42 49 54 20 |he mnemonic BIT | 00000600 77 68 69 63 68 0d 70 65 72 66 6f 72 6d 73 20 61 |which.performs a| 00000610 6e 20 41 4e 44 20 77 69 74 68 6f 75 74 20 63 68 |n AND without ch| 00000620 61 6e 67 69 6e 67 20 74 68 65 20 62 79 74 65 20 |anging the byte | 00000630 69 6e 20 74 68 65 0d 41 63 63 75 6d 75 6c 61 74 |in the.Accumulat| 00000640 6f 72 2e 20 20 49 74 20 64 6f 65 73 20 61 66 66 |or. It does aff| 00000650 65 63 74 20 74 68 65 20 66 6c 61 67 73 20 74 68 |ect the flags th| 00000660 6f 75 67 68 20 73 6f 20 62 72 61 6e 63 68 69 6e |ough so branchin| 00000670 67 0d 63 61 6e 20 62 65 20 64 6f 6e 65 20 6f 6e |g.can be done on| 00000680 20 74 68 65 20 72 65 73 75 6c 74 20 6f 66 20 61 | the result of a| 00000690 20 42 49 54 20 6f 70 65 72 61 74 69 6f 6e 2e 20 | BIT operation. | 000006a0 20 49 74 20 69 73 20 75 73 65 64 20 74 6f 0d 74 | It is used to.t| 000006b0 65 73 74 20 73 70 65 63 69 66 69 63 20 62 69 74 |est specific bit| 000006c0 73 20 28 68 65 6e 63 65 20 74 68 65 20 6d 6e 65 |s (hence the mne| 000006d0 6d 6f 6e 69 63 29 20 61 6e 64 20 74 6f 20 75 73 |monic) and to us| 000006e0 65 20 69 74 20 79 6f 75 0d 6c 6f 61 64 20 74 68 |e it you.load th| 000006f0 65 20 6d 61 73 6b 20 69 6e 74 6f 20 74 68 65 20 |e mask into the | 00000700 61 63 63 75 6d 75 6c 61 74 6f 72 20 61 6e 64 20 |accumulator and | 00000710 42 49 54 20 61 20 6d 65 6d 6f 72 79 20 6c 6f 63 |BIT a memory loc| 00000720 61 74 69 6f 6e 0d 6c 69 6b 65 20 74 68 69 73 3a |ation.like this:| 00000730 0d 0d 20 20 20 20 20 20 20 20 4c 44 41 20 23 31 |.. LDA #1| 00000740 0d 20 20 20 20 20 20 20 20 42 49 54 20 6e 75 6d |. BIT num| 00000750 62 65 72 20 20 20 20 20 20 20 5c 20 49 66 20 62 |ber \ If b| 00000760 69 74 20 30 20 69 73 20 31 20 74 68 65 6e 20 74 |it 0 is 1 then t| 00000770 68 65 20 7a 65 72 6f 0d 20 20 20 20 20 20 20 20 |he zero. | 00000780 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000790 20 5c 20 66 6c 61 67 20 77 69 6c 6c 20 62 65 20 | \ flag will be | 000007a0 63 6c 65 61 72 65 64 0d 20 20 20 20 20 20 20 20 |cleared. | 000007b0 42 4e 45 20 65 6c 73 65 77 68 65 72 65 20 20 20 |BNE elsewhere | 000007c0 20 5c 20 53 6f 20 62 72 61 6e 63 68 20 69 66 20 | \ So branch if | 000007d0 62 69 74 20 30 20 69 73 20 73 65 74 0d 0d 4f 52 |bit 0 is set..OR| 000007e0 20 69 73 20 74 68 65 20 6e 65 78 74 2c 20 77 69 | is the next, wi| 000007f0 74 68 20 74 68 65 20 6d 6e 65 6d 6f 6e 69 63 20 |th the mnemonic | 00000800 4f 52 41 2e 20 20 48 65 72 65 20 69 66 20 61 20 |ORA. Here if a | 00000810 70 61 72 74 69 63 75 6c 61 72 0d 62 69 74 20 6f |particular.bit o| 00000820 66 20 65 69 74 68 65 72 20 62 79 74 65 20 69 73 |f either byte is| 00000830 20 73 65 74 20 74 68 65 6e 20 74 68 61 74 20 62 | set then that b| 00000840 69 74 20 6f 66 20 74 68 65 20 72 65 73 75 6c 74 |it of the result| 00000850 20 77 69 6c 6c 0d 62 65 20 73 65 74 2e 0d 0d 20 | will.be set... | 00000860 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000870 20 20 30 20 20 31 20 20 20 20 20 20 20 20 20 20 | 0 1 | 00000880 20 30 20 30 52 20 30 20 3d 20 30 0d 20 20 20 20 | 0 0R 0 = 0. | 00000890 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 000008a0 20 20 20 20 20 20 20 20 20 20 20 20 20 20 31 20 | 1 | 000008b0 4f 52 20 30 20 3d 20 31 0d 20 20 20 20 20 20 20 |OR 0 = 1. | 000008c0 20 20 20 20 20 20 20 20 30 20 20 20 30 20 20 31 | 0 0 1| 000008d0 20 20 20 20 20 20 20 20 20 20 20 30 20 4f 52 20 | 0 OR | 000008e0 31 20 3d 20 31 0d 20 20 20 20 20 20 20 20 20 20 |1 = 1. | 000008f0 20 20 20 20 20 31 20 20 20 31 20 20 31 20 20 20 | 1 1 1 | 00000900 20 20 20 20 20 20 20 20 31 20 4f 52 20 31 20 3d | 1 OR 1 =| 00000910 20 31 0d 0d 54 68 69 73 20 69 73 20 75 73 65 64 | 1..This is used| 00000920 20 74 6f 20 6d 61 6b 65 20 73 75 72 65 20 74 68 | to make sure th| 00000930 61 74 20 62 69 74 73 20 6f 66 20 61 20 62 79 74 |at bits of a byt| 00000940 65 20 61 72 65 20 73 65 74 20 28 69 2e 65 2e 0d |e are set (i.e..| 00000950 74 68 65 20 72 65 76 65 72 73 65 20 6f 66 20 41 |the reverse of A| 00000960 4e 44 29 20 73 75 63 68 20 74 68 61 74 20 74 6f |ND) such that to| 00000970 20 73 65 74 20 74 68 65 20 74 6f 70 20 62 69 74 | set the top bit| 00000980 20 6f 66 20 61 20 6e 75 6d 62 65 72 0d 77 65 20 | of a number.we | 00000990 64 6f 20 74 68 69 73 3a 0d 0d 20 20 20 20 20 20 |do this:.. | 000009a0 20 20 20 4c 44 41 20 6e 75 6d 62 65 72 0d 20 20 | LDA number. | 000009b0 20 20 20 20 20 20 20 4f 52 41 20 26 38 30 20 20 | ORA &80 | 000009c0 20 20 20 20 20 20 20 5c 20 26 38 30 20 69 73 20 | \ &80 is | 000009d0 31 30 30 30 30 30 30 30 20 69 6e 20 62 69 6e 61 |10000000 in bina| 000009e0 72 79 0d 0d 61 6e 64 20 74 68 65 20 74 6f 70 20 |ry..and the top | 000009f0 62 69 74 20 77 69 6c 6c 20 62 65 20 73 65 74 20 |bit will be set | 00000a00 61 6e 64 20 74 68 65 20 72 65 73 74 20 77 69 6c |and the rest wil| 00000a10 6c 20 72 65 6d 61 69 6e 0d 75 6e 63 68 61 6e 67 |l remain.unchang| 00000a20 65 64 2e 0d 0d 54 68 65 20 66 69 6e 61 6c 20 6f |ed...The final o| 00000a30 70 65 72 61 74 6f 72 20 69 73 20 74 68 65 20 45 |perator is the E| 00000a40 78 63 6c 75 73 69 76 65 20 4f 52 2c 20 61 6e 64 |xclusive OR, and| 00000a50 20 45 4f 52 20 69 73 20 74 68 65 0d 6d 6e 65 6d | EOR is the.mnem| 00000a60 6f 6e 69 63 2e 20 20 48 65 72 65 20 61 20 62 69 |onic. Here a bi| 00000a70 74 20 69 6e 20 74 68 65 20 72 65 73 75 6c 74 20 |t in the result | 00000a80 69 73 20 73 65 74 20 69 66 20 74 68 61 74 20 62 |is set if that b| 00000a90 69 74 20 69 73 0d 73 65 74 20 69 6e 20 65 69 74 |it is.set in eit| 00000aa0 68 65 72 2c 20 62 75 74 20 6e 6f 74 20 62 6f 74 |her, but not bot| 00000ab0 68 2c 20 6f 66 20 74 68 65 20 6f 74 68 65 72 20 |h, of the other | 00000ac0 62 79 74 65 73 2e 0d 0d 20 20 20 20 20 20 20 20 |bytes... | 00000ad0 20 20 20 20 20 20 20 20 20 20 20 30 20 20 31 20 | 0 1 | 00000ae0 20 20 20 20 20 20 20 20 20 20 30 20 45 4f 52 20 | 0 EOR | 00000af0 30 20 3d 20 30 0d 20 20 20 20 20 20 20 20 20 20 |0 = 0. | 00000b00 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 | | 00000b10 20 20 20 20 20 20 20 20 31 20 45 4f 52 20 30 20 | 1 EOR 0 | 00000b20 3d 20 31 0d 20 20 20 20 20 20 20 20 20 20 20 20 |= 1. | 00000b30 20 20 20 30 20 20 20 30 20 20 31 20 20 20 20 20 | 0 0 1 | 00000b40 20 20 20 20 20 20 30 20 45 4f 52 20 31 20 3d 20 | 0 EOR 1 = | 00000b50 31 0d 20 20 20 20 20 20 20 20 20 20 20 20 20 20 |1. | 00000b60 20 31 20 20 20 31 20 20 30 20 20 20 20 20 20 20 | 1 1 0 | 00000b70 20 20 20 20 31 20 45 4f 52 20 31 20 3d 20 30 0d | 1 EOR 1 = 0.| 00000b80 0d 57 69 74 68 20 74 68 69 73 20 79 6f 75 20 63 |.With this you c| 00000b90 61 6e 20 72 65 76 65 72 73 65 20 74 68 65 20 73 |an reverse the s| 00000ba0 74 61 74 75 73 20 6f 66 20 61 20 66 6c 61 67 20 |tatus of a flag | 00000bb0 77 69 74 68 6f 75 74 0d 6b 6e 6f 77 69 6e 67 20 |without.knowing | 00000bc0 74 68 61 74 20 73 74 61 74 75 73 20 62 65 66 6f |that status befo| 00000bd0 72 65 68 61 6e 64 2e 20 20 49 20 63 61 6c 6c 20 |rehand. I call | 00000be0 69 74 20 66 6c 61 67 20 77 61 76 69 6e 67 20 77 |it flag waving w| 00000bf0 69 74 68 0d 45 4f 52 20 28 61 70 6f 6c 6f 67 69 |ith.EOR (apologi| 00000c00 65 73 20 74 6f 20 41 20 41 20 4d 69 6c 6e 65 29 |es to A A Milne)| 00000c10 2e 20 20 4c 65 74 27 73 20 73 61 79 20 79 6f 75 |. Let's say you| 00000c20 20 68 61 76 65 20 62 69 74 20 30 20 6f 66 20 61 | have bit 0 of a| 00000c30 0d 6e 75 6d 62 65 72 20 61 73 20 61 20 66 6c 61 |.number as a fla| 00000c40 67 20 61 6e 64 20 79 6f 75 20 77 61 6e 74 20 74 |g and you want t| 00000c50 6f 20 72 65 76 65 72 73 65 20 69 74 2c 20 74 68 |o reverse it, th| 00000c60 65 6e 20 79 6f 75 20 64 6f 0d 74 68 69 73 2e 0d |en you do.this..| 00000c70 0d 20 20 20 20 20 20 20 20 20 4c 44 41 20 6e 75 |. LDA nu| 00000c80 6d 62 65 72 0d 20 20 20 20 20 20 20 20 20 45 4f |mber. EO| 00000c90 52 20 31 20 20 20 20 20 20 20 20 20 20 20 5c 20 |R 1 \ | 00000ca0 31 20 69 73 2c 20 6f 66 20 63 6f 75 72 73 65 2c |1 is, of course,| 00000cb0 20 30 30 30 30 30 30 30 31 0d 0d 0d 45 4f 52 69 | 00000001...EORi| 00000cc0 6e 67 20 61 20 6e 75 6d 62 65 72 20 77 69 74 68 |ng a number with| 00000cd0 20 69 74 73 65 6c 66 20 61 6c 77 61 79 73 20 70 | itself always p| 00000ce0 72 6f 64 75 63 65 73 20 7a 65 72 6f 20 61 6e 64 |roduces zero and| 00000cf0 20 45 4f 52 69 6e 67 0d 77 69 74 68 20 26 46 46 | EORing.with &FF| 00000d00 20 72 65 76 65 72 73 65 73 20 74 68 65 20 73 65 | reverses the se| 00000d10 74 74 69 6e 67 73 20 6f 66 20 61 6c 6c 20 62 69 |ttings of all bi| 00000d20 74 73 2c 20 70 72 6f 64 75 63 69 6e 67 20 61 20 |ts, producing a | 00000d30 6f 6e 65 73 0d 63 6f 6d 70 6c 65 6d 65 6e 74 20 |ones.complement | 00000d40 6e 65 67 61 74 69 76 65 2e 20 20 41 20 6e 65 61 |negative. A nea| 00000d50 74 20 77 61 79 20 74 6f 20 70 72 6f 64 75 63 65 |t way to produce| 00000d60 20 74 68 65 20 6e 65 67 61 74 69 76 65 20 6f 66 | the negative of| 00000d70 0d 61 20 62 79 74 65 20 69 73 20 74 6f 20 45 4f |.a byte is to EO| 00000d80 52 20 69 74 20 77 69 74 68 20 26 46 46 20 28 32 |R it with &FF (2| 00000d90 35 35 29 20 61 6e 64 20 74 68 65 6e 20 61 64 64 |55) and then add| 00000da0 20 6f 6e 65 2e 20 20 43 68 65 63 6b 0d 62 61 63 | one. Check.bac| 00000db0 6b 20 74 6f 20 6d 6f 64 75 6c 65 20 32 20 74 6f |k to module 2 to| 00000dc0 20 73 65 65 20 77 68 79 20 69 74 20 77 6f 72 6b | see why it work| 00000dd0 73 2e 0d 0d 49 74 27 73 20 61 20 73 69 6d 70 6c |s...It's a simpl| 00000de0 65 20 6c 69 74 74 6c 65 20 70 69 65 63 65 20 6f |e little piece o| 00000df0 66 20 63 6f 64 65 20 69 6e 20 74 68 69 73 20 6d |f code in this m| 00000e00 6f 64 75 6c 65 2e 20 20 54 68 65 0d 41 53 43 49 |odule. The.ASCI| 00000e10 49 20 63 6f 64 65 73 20 61 72 65 20 61 72 72 61 |I codes are arra| 00000e20 6e 67 65 64 20 73 75 63 68 20 74 68 61 74 20 74 |nged such that t| 00000e30 68 65 20 63 6f 64 65 20 66 6f 72 20 61 20 6c 65 |he code for a le| 00000e40 74 74 65 72 20 41 4e 44 0d 26 44 46 20 77 69 6c |tter AND.&DF wil| 00000e50 6c 20 61 6c 77 61 79 73 20 70 72 6f 64 75 63 65 |l always produce| 00000e60 20 74 68 65 20 63 6f 64 65 20 66 6f 72 20 75 70 | the code for up| 00000e70 70 65 72 20 63 61 73 65 2c 20 61 6e 64 20 42 2f |per case, and B/| 00000e80 6f 73 62 30 38 0d 64 6f 65 73 20 73 69 6d 70 6c |osb08.does simpl| 00000e90 79 20 74 68 61 74 2e 20 20 48 6f 77 65 76 65 72 |y that. However| 00000ea0 2c 20 62 65 69 6e 67 20 73 69 6d 70 6c 65 20 69 |, being simple i| 00000eb0 74 20 64 6f 65 73 20 6e 6f 74 20 70 72 6f 64 75 |t does not produ| 00000ec0 63 65 0d 61 20 73 61 74 69 73 66 61 63 74 6f 72 |ce.a satisfactor| 00000ed0 79 20 72 65 73 75 6c 74 20 66 6f 72 20 61 6c 6c |y result for all| 00000ee0 20 74 68 65 20 6f 74 68 65 72 20 6b 65 79 73 20 | the other keys | 00000ef0 6f 6e 20 74 68 65 0d 6b 65 79 62 6f 61 72 64 2e |on the.keyboard.| 00000f00 20 20 54 68 69 73 20 61 70 70 72 6f 61 63 68 20 | This approach | 00000f10 69 73 20 6f 6e 65 20 6f 66 20 74 68 65 20 77 61 |is one of the wa| 00000f20 79 73 20 74 6f 20 6c 65 67 69 73 6c 61 74 65 0d |ys to legislate.| 00000f30 61 67 61 69 6e 73 74 20 70 65 6f 70 6c 65 20 77 |against people w| 00000f40 68 6f 20 74 75 72 6e 20 6f 66 66 20 28 6f 72 20 |ho turn off (or | 00000f50 6f 6e 29 20 74 68 65 20 43 41 50 53 20 4c 4f 43 |on) the CAPS LOC| 00000f60 4b 20 6f 6e 20 74 68 65 69 72 0d 6d 69 63 72 6f |K on their.micro| 00000f70 20 77 68 65 6e 20 79 6f 75 20 65 78 70 65 63 74 | when you expect| 00000f80 20 69 74 20 74 68 65 20 6f 74 68 65 72 20 77 61 | it the other wa| 00000f90 79 20 61 72 6f 75 6e 64 2e 20 20 54 68 65 20 6f |y around. The o| 00000fa0 74 68 65 72 0d 77 61 79 20 69 73 20 74 6f 20 75 |ther.way is to u| 00000fb0 73 65 20 4f 53 42 59 54 45 20 32 30 32 20 77 68 |se OSBYTE 202 wh| 00000fc0 69 63 68 20 73 69 6d 75 6c 61 74 65 73 20 70 72 |ich simulates pr| 00000fd0 65 73 73 69 6e 67 20 6f 66 20 74 68 65 0d 72 65 |essing of the.re| 00000fe0 6c 65 76 61 6e 74 20 6b 65 79 73 2e 20 20 54 68 |levant keys. Th| 00000ff0 65 20 6f 6e 6c 79 20 74 72 61 70 20 69 6e 20 74 |e only trap in t| 00001000 68 69 73 20 63 6f 64 65 20 69 73 20 74 6f 20 70 |his code is to p| 00001010 72 65 76 65 6e 74 20 61 0d 63 6f 6e 74 72 6f 6c |revent a.control| 00001020 20 63 6f 64 65 20 73 6e 65 61 6b 69 6e 67 20 74 | code sneaking t| 00001030 68 72 6f 75 67 68 20 77 69 74 68 6f 75 74 20 79 |hrough without y| 00001040 6f 75 20 65 78 70 65 63 74 69 6e 67 20 69 74 2e |ou expecting it.| 00001050 0d 0d 49 6e 20 74 68 65 20 6e 65 78 74 20 6d 6f |..In the next mo| 00001060 64 75 6c 65 20 49 27 6c 6c 20 67 69 76 65 20 79 |dule I'll give y| 00001070 6f 75 20 73 6f 6d 65 20 69 6e 66 6f 72 6d 61 74 |ou some informat| 00001080 69 6f 6e 20 61 62 6f 75 74 20 74 68 65 0d 6f 74 |ion about the.ot| 00001090 68 65 72 20 6f 70 65 72 61 74 69 6e 67 20 73 79 |her operating sy| 000010a0 73 74 65 6d 20 72 6f 75 74 69 6e 65 73 20 69 6e |stem routines in| 000010b0 20 74 68 65 20 42 42 43 20 4d 69 63 72 6f 20 74 | the BBC Micro t| 000010c0 6f 20 64 6f 20 77 69 74 68 0d 66 69 6c 65 20 68 |o do with.file h| 000010d0 61 6e 64 6c 69 6e 67 20 61 6e 64 20 74 68 65 20 |andling and the | 000010e0 6c 69 6b 65 2c 20 61 6e 64 20 77 65 27 6c 6c 20 |like, and we'll | 000010f0 75 73 65 20 6f 6e 65 20 6f 66 20 74 68 65 6d 20 |use one of them | 00001100 74 6f 0d 70 72 69 6e 74 20 6f 75 74 20 61 20 6c |to.print out a l| 00001110 69 73 74 20 6f 66 20 74 68 65 20 52 4f 4d 53 20 |ist of the ROMS | 00001120 69 6e 20 79 6f 75 72 20 6d 61 63 68 69 6e 65 2e |in your machine.| 00001130 0d 0d 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e |................| 00001140 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e |................| * 00001160 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 2e 0d 0d |................| 00001170 50 53 3a 20 20 54 68 61 6e 6b 20 79 6f 75 20 66 |PS: Thank you f| 00001180 6f 72 20 79 6f 75 72 20 66 65 65 64 62 61 63 6b |or your feedback| 00001190 2e 20 20 49 27 6d 20 70 61 72 74 69 63 75 6c 61 |. I'm particula| 000011a0 72 6c 79 20 67 72 61 74 65 66 75 6c 0d 74 6f 20 |rly grateful.to | 000011b0 50 65 74 65 72 20 56 69 6e 63 65 20 66 6f 72 20 |Peter Vince for | 000011c0 68 69 73 20 63 6f 6e 73 74 72 75 63 74 69 76 65 |his constructive| 000011d0 20 63 72 69 74 69 63 69 73 6d 20 61 6e 64 20 73 | criticism and s| 000011e0 70 65 6c 6c 69 6e 67 0d 63 68 65 63 6b 65 72 20 |pelling.checker | 000011f0 61 6e 64 20 49 20 68 6f 70 65 20 74 68 61 74 20 |and I hope that | 00001200 6d 79 20 6f 64 64 20 74 79 70 6f 20 61 6e 64 20 |my odd typo and | 00001210 28 72 61 72 65 21 29 20 62 72 61 69 6e 20 63 6c |(rare!) brain cl| 00001220 6f 67 0d 68 61 76 65 20 6e 6f 74 20 63 6f 6e 66 |og.have not conf| 00001230 75 73 65 64 20 79 6f 75 2e 0d |used you..| 0000123a
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