By now we've covered the basics of every command in the assembler; in this final installment we show the full capabilities of the .text and .data commands as we produce a more sophisticated set of Commodore 64 header files.

Our print'str routine in accesses memory locations $10 and $11 directly. We'd prefer to have symbolic names for them. This reprises our concerns back in when we concluded that we wanted two separate program counters. Now we realize that we really need three; one for the text, one for the data, and one for the zero page data. And if we're going to allow three, we really should allow any number.

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 $80 The final source file is .