Section 2 - Commodore Computers

May. 1982. Issue 24 COMPUTEI 111
May. 1982. Issue 24 COMPUTE! 111
Equal as we would like. F. R. scratches his head
and makes a note to fix x that up later somehow.
All AH we have left is a bland PRINT statement
on line 260. Print what? at' A Return character, r, off
course, to end the line. F. R. knows that this characte
r is a decimal thirteen or hexadecimal 00, so
acter is a decimal thirteen or hexadecimal OD, so
he codes LOA LDA ##$0D $00 to bring it into the A register
and then JSR $FFO $FFDII to print it as before.
The coding job's done, and F. R. notes ddown
the last instruction: RTS, Return from Subroutine,
the last instruction: RTS, Return from Subroutine,
which will cause the machine language program to
return to the BASIC program which called it. He
sits back. Then he notices that he's scribbled the
sits back. Then he notices that he's scribbled the
whole thing on the back of his subscription renewal
to COMPUTE! (the magazine gets a lot of programs
to COMPUTE! (the magazine gets a lot of programs
that way), and decides to make a copy.
This is the program he transcribes onto a
stenographer's note pad. The instructions are to
the right off the vertical center line; the two notes,
YLOOP and SKIP are just to the left. T he re's
YLOOP and SKIP are just to the left. There's
plenty of space to the left; he'll be using that later.
Here is what his coding looks like:
Here is what his coding looks like:
LOX LDX #$30
LOY LDY #$01
YLOOP INX
YLOOP
INX
CPX ##$3A
BNE SKIP
LDX LOX #$30
SKIP
TXA
JSR $FFD2
INY
CPY $0300
BCC YLOOP
LOA LDA #$00 #$0D
JSR
$FFD2
RTS
You should be able to relate this program to the
BASIC program previouslyp given. One important
point: where the "#"" sign is used -— you may call it
hash, h. nutnbers. numbers, or pounds sign -— we want thee progran1
gram to use the actual value. Where it is not used. used, we
want the program to use the contents of an address.
The "#"" sign signals immediate iate addressing -— not
really an address at all, , butthe the actual value.
F. R. Vescent's Vescelll's program m is not readyy to run yet.
He has written it in Source (or Assembler) language
for his own convenience. nience. The computer can run
onlyy Object (or Machine) language. He must translate
from Source to Object language by knowing the
codes. This s translation job is called assembly. We'll
tackle it next time. ©
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