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SBB -- Integer Subtraction with Borrow

Opcode       Instruction       Clocks  Description

1C  ib       SBB AL,imm8       2       Subtract with borrow immediate byte
                                       from AL
1D  iw       SBB AX,imm16      2       Subtract with borrow immediate word
                                       from AX
1D  id       SBB EAX,imm32     2       Subtract with borrow immediate
                                       dword from EAX
80  /3 ib    SBB r/m8,imm8     2/7     Subtract with borrow immediate byte
                                       from r/m byte
81  /3 iw    SBB r/m16,imm16   2/7     Subtract with borrow immediate word
                                       from r/m word
81  /3 id    SBB r/m32,imm32   2/7     Subtract with borrow immediate
                                       dword from r/m dword
83  /3 ib    SBB r/m16,imm8    2/7     Subtract with borrow sign-extended
                                       immediate byte from r/m word
83  /3 ib    SBB r/m32,imm8    2/7     Subtract with borrow sign-extended
                                       immediate byte from r/m dword
18  /r       SBB r/m8,r8       2/6     Subtract with borrow byte register
                                       from r/m byte
19  /r       SBB r/m16,r16     2/6     Subtract with borrow word register
                                       from r/m word
19  /r       SBB r/m32,r32     2/6     Subtract with borrow dword register
                                       from r/m dword
1A  /r       SBB r8,r/m8       2/7     Subtract with borrow byte register
                                       from r/m byte
1B  /r       SBB r16,r/m16     2/7     Subtract with borrow word register
                                       from r/m word
1B  /r       SBB r32,r/m32     2/7     Subtract with borrow dword register
                                       from r/m dword

Operation

IF SRC is a byte and DEST is a word or dword
THEN DEST := DEST - (SignExtend(SRC) + CF)
ELSE DEST := DEST - (SRC + CF);

Description

SBB adds the second operand (DEST) to the carry flag (CF) and subtracts the result from the first operand (SRC). The result of the subtraction is assigned to the first operand (DEST), and the flags are set accordingly.

When an immediate byte value is subtracted from a word operand, the immediate value is first sign-extended.

Flags Affected

OF, SF, ZF, AF, PF, and CF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


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