Programming with Ophis | ||
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The .data and .text commands can take a label name after them—this names a new segment. We'll define a new segment called zp (for "zero page") and have our zero-page variables be placed there. We can't actually use the default origin of $0000 here either, though, because the Commodore 64 reserves memory locations 0 and 1 to control its memory mappers:
.data zp .org $0002 |
Now, actually, the rest of the zero page is reserved too: locations $02-$8F are used by the BASIC interpreter, and locations $90-$FF are used by the KERNAL. We don't need the BASIC interpreter, though, so we can back up all of $02-$8F at the start of our program and restore it all when we're done.
In fact, since we're disablng BASIC, we can actually also swap out its ROM entirely and get a contiguous block of RAM from $0002 to $CFFF:
.scope ; Cache BASIC zero page at top of available RAM ldx #$8e * lda $01, x sta $cf81, x dex bne - ; Swap out the BASIC ROM for RAM lda $01 and #$fe ora #$06 sta $01 ; Run the real program jsr _main ; Restore BASIC ROM lda $01 ora #$07 sta $01 ; Restore BASIC zero page ldx #$8e * lda $cf81, x sta $01, x dex bne - ; Back to BASIC rts _main: ; _main points at the start of the real program, ; which is actually outside of this scope .scend |
Our print'str routine is then rewritten to declare and use a zero-page variable, like so:
; PRINTSTR routine. Accumulator stores the low byte of the address, ; X register stores the high byte. Destroys the values of $10 and ; $11. .scope .data zp .space _ptr 2 .text printstr: sta _ptr stx _ptr+1 ldy #$00 _lp: lda (_ptr),y beq _done jsr chrout iny bne _lp _done: rts .scend |
Also, we ought to put in an extra check to make sure our zero-page allocations don't overflow, either:
.data zp .checkpc $90 |
The final source file is hello7.oph.
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