SBC = Subtraction with Carry (8-Bits)
| Hex | Mnemonic | Cycles |
|---|---|---|
| 18 | SBC A, A | 8 |
| 19 | SBC A, B | 8 |
| 1A nn | SBC A, #nn | 8 |
| 1B | SBC A, [HL] | 8 |
| 1C nn | SBC A, [N+#nn] | 12 |
| 1D nn nn | SBC A, [#nnnn] | 16 |
| 1E | SBC A, [X] | 8 |
| 1F | SBC A, [Y] | 8 |
| CE 18 ss | SBC A, [X+#ss] | 16 |
| CE 19 ss | SBC A, [Y+#ss] | 16 |
| CE 1A | SBC A, [X+L] | 16 |
| CE 1B | SBC A, [Y+L] | 16 |
| CE 1C | SBC [HL], A | 16 |
| CE 1D nn | SBC [HL], #nn | 20 |
| CE 1E | SBC [HL], [X] | 20 |
| CE 1F | SBC [HL], [Y] | 20 |
Execute
#nn = Immediate unsigned 8-Bits
#ss = Immediate signed 8-Bits
A = Register A
B = Register B
[N+#nn] = Memory: (I shl 16) or (N shl 8) or #nn
[HL] = Memory: (I shl 16) or HL
[X] = Memory: (XI shl 16) or X
[Y] = Memory: (YI shl 16) or Y
[#nnnn] = Memory: (I shl 16) or #nnnn
[X+#ss] = Memory: (XI shl 16) or (X + #ss)
[Y+#ss] = Memory: (YI shl 16) or (Y + #ss)
[X+L] = Memory: (XI shl 16) or (X + signed(L))
[Y+L] = Memory: (YI shl 16) or (Y + signed(L))
; SBC Ds, Sc
;
; Ds = Destination
; Sc = Source
; (If flags BCD=0 and NIBBLE=0)
Ds = Ds - Sc - Carry
;------------------------------------------------
; (If flags BCD=0 and NIBBLE=1)
Ds & 0x0F = (Ds & 0x0F) - (Sc & 0x0F) - Carry
;------------------------------------------------
; (If flags BCD=1 and NIBBLE=0)
IF (((Ds & 15) - (Sc & 15) - Carry) >= 10) THEN ; Note: Should be an unsigned check
Ds = Ds - Sc - Carry - 6
ELSE
Ds = Ds - Sc - Carry
ENDIF
IF (Ds >= 0xA0) Ds = Ds - 0x60
; NOTE: This isn't accurate, there's some weird effect on a strange condition
;------------------------------------------------
; (If flags BCD=1 and NIBBLE=1)
IF (((Ds & 15) - (Sc & 15) - Carry) >= 10) THEN ; Note: Should be an unsigned check
Ds & 0x0F = (Ds & 0x0F) - (Sc & 0x0F) - Carry - 6
ELSE
Ds & 0x0F = (Ds & 0x0F) - (Sc & 0x0F) - Carry
ENDIF
Description
8-Bits Source and Carry subtracts to the 8-Bits Destination.
Conditions
If flags BCD=0 and NIBBLE=0
Zero: Set when result is 0
Carry: Set when result is < 0
Overflow: Set when result overflow 8-bits signed range (< -128 OR > 127)
Sign: Set when bit 7 of the result is 1
If flags BCD=0 and NIBBLE=1
Zero: Set when result is 0
Carry: Set when result is < 0
Overflow: Set when result overflow 4-bits signed range (< -16 OR > 15)
Sign: Set when bit 3 of the result is 1
If flags BCD=1 and NIBBLE=0
Zero: Set when result is 0
Carry: Set when result is < 0
Overflow: Always Reset
Sign: Always Reset
If flags BCD=1 and NIBBLE=1
Zero: Set when result is 0
Carry: Set when result is < 0
Overflow: Always Reset
Sign: Always Reset
Examples
; A = 0x55
; F = (Carry=1)
SBC A, $80
; A = 0xD4 (0x55 - 0x80 - 0x01 = 0x(1)D4)
; F = (Zero=0):(Carry=0):(Overflow=1):(Sign=1)
; B = 0x31
; A = 0xCF
; F = (Carry=0)
SBC A, B
; B = 0x31
; A = 0x9E (0xCF - 0x31 - 0x00 = 0x(0)9E)
; F = (Zero=0):(Carry=0):(Overflow=0):(Sign=0)
; ( BCD = 1, NIBBLE = 0 )
; A = 0x10
; F = (Carry=1)
SBC A, $01
; A = 0x08 (10 - 01 - 01 = 08)
; F = (Zero=0):(Carry=0):(Overflow=0):(Sign=0)
; ( BCD = 0, NIBBLE = 1 )
; A = 0x3F (Note, in "nibble mode" the high nibble is always discarded)
; F = (Carry=1)
SBC A, $01
; A = 0x0D (0xF - 0x1 - 0x1 = 0x(0)D)
; F = (Zero=0):(Carry=0):(Overflow=0):(Sign=0)