Hi,
I've put the question together with some more input at
http://pofo.de/P8000/problems.php
Yesterday I solved another problem which - I thought - was a bit simmilar
I had the following ASM code:
ldk r2,#0
ldb rl2,_u+1060
ld r3,r2
neg r3
add r3,#256
ldb rh3,rl3
clrb rl3
ld _u+48,r3
so i created the following C code out of it:
u.u_count = (-u.u_segmts[NUSEGS-1].sg_limit+0x100)<<8;
but this compiled to this ASM code:
ldk r2,#0
ldb rl2,_u+1060
neg r2
add r2,#256
ldb rh2,rl2
clrb rl2
ld _u+48,r2
As you can see the copy of r2 to r3 and the further processing with r3 is
missing here. I also thought "who the fuck would write such an C-code,
the code must look different". but I did not found the solution what
could have been written in the C code until I've talked today with a
colleague of mine at work about ASM and my problems. He isn't familar
with Z80(00)-ASM but he used to program ASM years ago with his C64. We
took the ASM code and simulated it with values:
_u+1060 contains 15 and is loaded to rl2 15 (0x000F / 00000000 00001111)
this gets negated (2 complement formed) -15 (0xFFF1 / 11111111 11110001)
to that, 256 gets added 241 (0x00F1 / 00000000 11110001)
this is 8 bit rightshiftet 61696 (0xF100 / 11110001 00000000)
the result gets loaded into _u+48
He then got the idea that all this could be aritmetical written
as ((256 - 15)*256) because -15+256 is == 256-15 and rightshifts are done
aritmetically by multiplying the value with 256. It could have also been
done by having 256? - 256*x. This was great. With that information I wrote
in C:
u.u_count = (256-u.u_segmts[NUSEGS-1].sg_limit)<<8;
And this generated the same ASM code as in the original code
problem solved :)
--
Oliver Lehmann
http://www.pofo.de/
http://wishlist.ans-netz.de/