self-modifying code HELP!
Entry posted by EricBall
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My Propeller video driver is almost done, but I've run into a snag. The following is a snippet from the pixel byte to long lookup routine.
SHR sprgfx1, #8 ' shift byte into position MOV sprbyte, sprgfx1 AND sprbyte, #$0FF wz MOVS :byte6, sprbyte ' update source pointer MOVD :byte6, sprdata ' update destination pointer ADD sprdata, #1 :byte6 IF_NZ MOV sprdata, sprbyte ' copy color lookup table entry to lineRAM SHR sprgfx1, #8 wz MOVS :byte7, sprgfx1 MOVD :byte7, sprdata :byte7 IF_NZ MOV sprdata, sprbyte ' won't work due to pipeline DJNZ count2, #:nxtspr ' decrement active sprite counter JMP #doblank ' max number of active sprites reached :nxtspr ADD sprptr, #4 DJNZ count1, #:loop ' 48+16+223 = 287 (288) max cycles / sprite doblank
What's going on here is some self-modifying code 'cause the Propeller doesn't have any indirect addressing. My problem is the propeller is moderately pipelined, so the next instruction is fetched before the current instruction has been executed. So :byte7 MOV won't reflect the MOVD as it currently stands. For all of the other cases I've been able to slip the ADD instruction back before the MOV, but I don't have that option for :byte7. I could just put a NOP in there, but that adds 4 cycles to the loop, which effectively adds 16 cycles due to external memory timing. (ick!) Other options:
1. Slide back the DJNZ. But then I need to copy :byte7 so it gets executed before the JMP. That's 3 opcodes (again due to the pipeline) and I'd like to only add max 2 for other reasons.
2. Roll up the byte to long translation. Code space efficient, but more cycles.
Any other suggestions?
(The wz / IF_NZ are to make color 0 transparent. The condition codes are under application control and every instruction may be optionally executed. So wz sets/clears the Zero condition code and IF_NZ executes the MOV only if the Zero condition code is cleared, i.e. the value of sprbyte is non-zero.)
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