Re: [cc65-devel] HuC6280 zero-page addressing problem.

[Damian Y. <te...@gm...>]

On Wed, Jul 6, 2016 at 12:48 AM, Greg King <gr...@us...> wrote:
>
> On 2016-06-25 2:07 PM, John Brandwood wrote:
> >
> > The obvious question is ... would you allow the programmer to set the
> > direct-page to a symbol whose location is not resolved until link-time?
>
> ld65 knows nothing about op-codes; therefore, it cannot change
> instructions.  .DPAGE's operand will need to be an assembler constant.

A non-constant argument to .DPAGE can still be useful in theory if the
*difference* between each affected operand and the .DPAGE operand is
constant. Or if the program forces direct page, such as through a z:
prefix or a direct-page-only addresing mode (such as (d),Y), it can
assemble the instruction and add a link-time assertion that the
operand is 0-255.

.p816
.import someabsvar, another
quarterin = someabsvar + $40
halfwayin = someabsvar + $80

pea someabsvar
pld
.dpage someabsvar

; These should assemble to DP opcodes because
; (quarterin - someabsvar) is constant at assembly time
lda z:quarterin
lda (quarterin),y
lda quarterin

; These should assemble to DP opcodes followed by
; an assertion that (another - someabsvar) is in 0-255
lda z:another
lda (another),y

; This should assemble to an absolute opcode because
; the difference between the symbols can't be determined
; at assembly time
lda another