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)
    

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