create a signed 16 bit offset from 8 bit

[BSC]

Just wanted to have some Z80 NOP squeezing chit chat

I am doing this to convert a signed 8 bit value in A into a signed 16 bit value in DE:

Code: [Select]

    ld e,a      ; 1 copy lo-byte
    rlca        ; 2 rotate sign into carry flag
    ld a,0      ; 4 subtract from 0 ..
    sbc a       ; 5 with carry, effectively subtracting the carry flag from 0 => a is either 0 or -1
    ld d,a      ; 6 copy as hi-byte
    ; => signed offset value in de

Is there a faster way to do that? If not: :-P

[TFM]

Is there a faster way to do that? If not: :-P

Yes...

Code: [Select]

   
ld   e,a    ; 1 copy lo-byte   
rlca        ; 2 rotate sign into carry flag   
sbc  a,a    ; 3 with carry, effectively subtracting the carry flag from 0 => a is either 0 or -1     
ld   d,a    ; 4 copy as hi-byte    ; => signed offset value in de


[BSC]

Yes...

Code: [Select]

   
ld   e,a    ; 1 copy lo-byte   
rlca        ; 2 rotate sign into carry flag   
sbc  a,a    ; 3 with carry, effectively subtracting the carry flag from 0 => a is either 0 or -1     
ld   d,a    ; 4 copy as hi-byte    ; => signed offset value in de


It seems that you are right ..

[TFM]

Auch ein blindes Huhn trinkt mal einen Korn oder so