4/17/98 6-161 Addressing Modes and Data Types
SUBB Subtract with Borrow
Syntax: SUBB dest, src
Operation: dest <- dest - src - C
Description: Performs a twos complement binary addition of the source operand and the
previously generated carry bit (borrow) with the destination operand. The result is stored in the
destination operand.The source data is not affected by the operation.
If the carry from previous operation is zero (C = 0, i.e., Borrow = 1), the result is exact difference
of the operands; if it is one (C = 1, i.e., Borrow = 0), the result is 1 less than the difference in
operands.
This form of subtraction is intended to support multiple-precision arithmetic. For this use, the carry
bit is first reset, then SUBB is used to add the portions of the multiple-precision values from least-
significant to most-significant.
Size: Byte-Byte, Word-Word
Flags Updated: C, AC, V, N, Z
SUBB Rd, Rs
Bytes: 2
Clocks: 3
Operation: (Rd) <-- (Rd) - (Rs) - (C)
Encoding:
SUBB Rd, [Rs]
Bytes: 2
Clocks: 4
Operation: (Rd) <-- (Rd) - ((WS:Rs)) - (C)
Encoding:
0 0 1 1 SZ 0 0 1 d d d d s s s s
0 0 1 1 SZ 0 1 0 d d d d 0 s s s
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