Tuesday, Debtember 19, 2023
Timing code from inside an assembler
Back in March, I wrote about some 6809 optimizations where I counted CPU cycles by hand. I came across that code the other day and thought to myself, my 6809 emulator counts cycles, and I've embedded it into my 6809 assembler—how hard could it be to time code in addition to testing it?
Turns out—not terribly hard.
I added an option to the .TRON directive to count cycles instead of printing code execution and have the .TROFF directive print the cycle count
(indirectly,
since the code isn't run until the end of the second pass of the assembler).
Then I wrote up a few tests:
.test "ROM-RAMx1-byte" ldx #$8000 .tron timing r2r1 sta $FFDE lda ,x sta $FFDF sta ,x+ cmpx #$FF00 bne r2r1 .troff rts .endtst ;***************************************************************** .test "ROM-RAMx2-byte" ldx #$8000 .tron timing r2r2 sta $FFDE ldd ,x sta $FFDF std ,x++ cmpx #$FF00 bne r2r2 .troff rts .endtst ;***************************************************************** .test "ROM-RAMx4-byte" ldx #$8000 .tron timing r2r4 sta $FFDE ldd ,x ldu 2,x sta $FFDF std ,x++ stu ,x++ cmpx #$FF00 bne r2r4 .troff rts .endtst ;***************************************************************** .test "ROM-RAMx8-byte" savesp equ $0100 orcc #$50 sts savesp lds #$FF00 - 8 .tron timing r2r8 sta $FFDE puls u,x,y,d sta $FFDF pshs u,x,y,d leas -8,s cmps #$8000 - 8 bne r2r8 .troff lds savesp andcc #$AF rts .endtst
And upon running it:
GenericUnixPrompt% a09 -ftest r2r.asm ROM-RAMx1-byte:13: cycles=877824 ROM-RAMx2-byte:28: cycles=487680 ROM-RAMx4-byte:45: cycles=357632 ROM-RAMx8-byte:64: cycles=199136
The results match what I calculated by hand, so that's good. It also found a bug in the emulator—I had the wrong cycle count for one of the instructions. It's a bit scary how easy it has become to test 6809 assembly code now that I can do much of it when assembling the code.
