Assembly Language Hello World in color using direct writes to screen RAM for the Commodoare 64


Instead of using the built in routines to output characters to the screen, this version of the Hello World program shows how you can write characters directly to screen memory.

The Commodore 64 has two seperate screen RAM locations that affect what is displayed. One memory area starts at hex $0400 (decimal 1024) which is used to store the character to be displayed. The other area is the color RAM, which determines what color of the the corresponding character in screen RAM is displayed in. Color RAM starts at hex $D800 (decimal 55296).

For example if you type the following BASIC pokes in you will see a green letter A.

POKE 1024,1: POKE 55296,5

if you add one to 1024 the next character will be placed in the screen position to the right of the A and the corresponding color RAM location will be 55297.

Note the variation on clearing the screen in this version, which uses the routin at $e544 to clear the screen rather than the alternative which could have been used.

The BASIC equivalent to the screen clearing routine call is

SYS58692

Which in assembly is just

jsr $e544

Here is the source for the full program to store a text string directly in screen RAM.

!to “hello2.prg”,cbm
; comment out the firtst line if your assembler doens’t like it

; filename: hello2.asm

; Assmbly Language Hello World in color using direct writes to screen RAM
; for the Commodoare 64

; assemble with acme assembler or modify to for your particular assember
; acme hello.asm

; to run in C64 BASIC and
; load the compiled file:

; LOAD”HELLO2.PRG”,8,1
; SYS32768

*=$8000 ; start address 32768
scrnmem = $0400 ; start of screen memory
colormem = $d800 ; start of color memory

clrscrn = $e544 ; clear screen routine

main
clearscrn ; clear the entire screen
jsr clrscrn

ldx #$00 ; char ptr
ldy #$00 ; use Y to store current color

loop ; output text string (in this case “HELLO WORLD”)
lda hello,x ; load ptr to next char
cmp #$0 ; check if byte is 0 to terminate string
beq done

outchar
and #$3f ; correction because for poke A is 1 and not 65.
sta scrnmem,x ; store character directly in screen memory

; change the color for each char (just for fun)
chgcolor
iny ; next color
cpy #$06 ; skip the standard background color
; otherwise we wont see the character
beq chgcolor

tya
sta colormem,x ; colorize character
inx ; add one to char ptr

continue ; keep going until we have processed the entire string
clv
bvc loop ; branch always – two opcode relocatable version of jmp

done ; return to BASIC
rts

; null terninated string

hello !text “HELLO WORLD”,0
; hello .byte “HELLO WORLD”,0 ; for some assemblers use this instead

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s