Signed 16-bit right shift question with -1 shift check

[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.

[dmsc]

Hi!

JimBoyd wrote:

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
}

[JimBoyd]

dmsc wrote:

You are right, this should work:

It does. As Mike said, well done!

[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

[paul_nicholls]

Thanks for all the answers everyone, much appreciated

I think I'm good now

cheers,
Paul