Commodore VIC-20 - Writing Your First Program

WRITING YOUR FIRST PROGRAM '
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If you have used the POKE statement in BASIC to put characters
onto the screen, you will be aware that the character codes for
POKEing are different to CBM ASCII character values. For
example, if you enter:
PRINT ASC("A") (and press <RETURN> )
The VIC 20 will respond with:
65
READY.
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However, to put an "A" onto the screen by POKEing, the code is
1. Since the screen memory starts at 7680 (decimal), or 4096 if you
have 8K or more of expansion memory, by entering:
<CLR> (To clear the screen)
POKE 7680,1 (and<RETURN> ) (NOTE: POKE 4096,1 on a
VIC 20
with 8K or more of
expansion memory) ■
The "P" in the POKE statement should now be an "A." We will
now do this in machine language. Type the following in VICMon:
(Your cursor should be flashing alongside a "." right now.)
.A 1400 LDA #$01 (and press <RETURN> )
The VIC will prompt you with:
.A 1402 '
Type:
.A 1402 STA S1E00 (or STA S1000 on a VIC 20
with 8K or more of expansion memory)
The STA instruction stores the contents of the accumulator in a
specified memory location. The VIC will now prompt you with:
.A 1405 "
Now enter:
.A 1405 BRK
Clear the screen, and type:
G 1400
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