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Re: Signed 16-bit right shift question with -1 shift check
Posted: Tue Oct 09, 2018 7:21 pm
by JimBoyd
No, adding 1 before shifting doesn't work. If the dividend is -1, it is set to zero. The carry flag is still set from the test of the sign bit so when the number is shifted, the result returned is -32768.
Re: Signed 16-bit right shift question with -1 shift check
Posted: Wed Oct 10, 2018 9:15 pm
by dmsc
Hi!
No, adding 1 before shifting doesn't work. If the dividend is -1, it is set to zero. The carry flag is still set from the test of the sign bit so when the number is shifted, the result returned is -32768.
You are right, this should work:
Code: Select all
.macro div2_16bit(address) {
ldy address + 1
bpl ok
inc address +0
bne ok
iny
ok:
cpy #$80
tya
ror
ror address + 0
sta address + 1
}
Re: Signed 16-bit right shift question with -1 shift check
Posted: Fri Oct 12, 2018 8:56 pm
by JimBoyd
You are right, this should work:
It does. As Mike said, well done!
Re: Signed 16-bit right shift question with -1 shift check
Posted: Mon Oct 15, 2018 9:53 pm
by Chromatix
Here's my attempt, which just special-cases the -1 case:
Code: Select all
.Div16_2
LDA address
AND address+1
INC A ; turns 0xFFFF into zero, anything else will be non-zero
BNE Shift16
STA address ; explicitly store a zero result
STA address+1
BEQ End
.Shift16
LDA address+1
ASL A
ROR address+1
ROR address
.End
Re: Signed 16-bit right shift question with -1 shift check
Posted: Tue Oct 16, 2018 6:50 am
by paul_nicholls
Thanks for all the answers everyone, much appreciated
I think I'm good now
cheers,
Paul