Calculator Community > The Axe Parser Project
Assembly Programmers - Help Axe Optimize!
DJ Omnimaga:
Hopefully, though, maybe someone will write a 84+ emu for the Nspire to replace the current one, now that Ndless is out :P
Quigibo:
Does anyone know an efficient way to do signed multiplication for a 2's compliment system? I can't find any tutorials on the internet. My naive method is to remove and keep track of the sign bit for each term, multiply the positive versions together, and then add the new sign bit. Is there a better method?
This is what I'm using to get the sign bit out of de and keep track of it with the 'b' register. b starts at zero, so I can use bit 0 of b as the new sign bit if I repeat this for hl.
--- Code: --- bit 7,d
jr z,__MulSNotNeg
inc b
xor a
sub e
ld e,a
sbc a,a
sub d
ld d,a
__MulSNotNeg:
--- End code ---
I would have to do the same thing for hl and then again at the end when I need to put the bit back so it seems like a lot of extra code...
calc84maniac:
No need at all. 16-bit * 16-bit -> 16-bit will give the same result for unsigned and signed arithmetic. Problem solved! ;)
Seriously, try multiplying some signed values in your parser and you will get the right results.
Edit:
Scratch that, I just tried it myself. What is your normal multiplication routine?
Edit2:
I just disassembled it, and I think you are only doing an 8-bit * 16-bit multiplication. That could explain the bad outputs.
Galandros:
I have some ideas but I don't know how well it will be implemented:
There are some shift instructions that preserve the sign bit, for example sra. And sla is other arithmetic shift... Probably this isn't useful or as fast as other methods.
When the multiplication is finished you can set the correct sign based in the inputs.
You can probably use the sign flag to optimize.
neg is equivalent to cpl / inc a. Dunno how this goes into 16-bit pair registers.
calc84maniac:
Here is your original multiplication routine:
--- Code: --- xor a
or d
jr nz,$+3
ex de,hl
ld a,l
ld hl,0
_multloop:
rra
jr nc,$+3
add hl,de
sla e
rl d
or a
jr nz,_multloop
ret
--- End code ---
Here is my modified signed version, only 8 extra bytes (all in the overhead). It multiplies two signed values, one of which is between -256 and 255.
--- Code: --- ld a,d
rrca
cp d
jr nz,$+3
ex de,hl
xor a
inc h
jr nz,$+3
sub e
ld h,a
ld a,l
ld l,0
or a ;Returns if multiplying by 0 or -256, also resets carry flag
ret z
_multloop:
rra
jr nc,$+3
add hl,de
sla e
rl d
or a
jr nz,_multloop
ret
--- End code ---
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