DR805x Instructions set details
- 72 -
3.40. SUBB
Instruction:
Function:
SUBB A, <src-byte>
Subtract with borrow
Description: SUBB subtracts the indicated variable and the carry flag together from
the accumulator, leaving the result in the accumulator. SUBB sets the
carry (borrow) flag if a borrow is needed for bit 7, and clears C
otherwise. (If C was set before executing a SUBB instruction, this
indicates that a borrow was needed for the previous step in a multiple
precision subtraction, so the carry is subtracted from the accumulator
along with the source operand). AC is set if a borrow is needed for bit 3,
and cleared otherwise. OV is set if a borrow is needed into bit 6 but not
into bit 7, or into bit 7 but not bit 6.
When subtracting signed integers OV indicates a negative number
produced when a negative value is subtracted from a positive value, or
a positive result when a positive number is subtracted from a negative
number.
The source operand allows four addressing modes: register, direct,
register-indirect, or immediate.
3.40.1.
SUBB A, RN
Operation: (PC) ← (PC) + 1
(A) ← (A) - (C) - (Rn)
Bytes:
1
Cycles:
2
Encoding:
1
0
0
1
1
r
r
r
3.40.2.
SUBB A, DIRECT
Operation: (PC) ← (PC) + 2
(A) ← (A) - (C) - (direct)
Bytes:
Cycles:
2
3
Encoding:
1
0
0
1
0
1
0
1
direct address
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