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Subtract From Accumulator With Borrow
SUBB
7-179
Assembly Language Instructions
Cycles for a Repeat (RPT) Execution of an SUBB Instruction
Program
Operand
ROM
DARAM SARAM External
DARAM n n n n+p
SARAM n n n, n+1
†
n+p
External n+nd n+nd n+nd n+1+p+nd
†
If the operand and the code are in the same SARAM block
Example 1 SUBB DAT5 ;(DP = 8: addresses 0400h–047Fh)
Before Instruction After Instruction
Data Memory Data Memory
405h 06h 405h 06h
ACC 0 06h ACC 0 0FFFFFFFFh
CC
Example 2 SUBB *
Before Instruction After Instruction
ARP 6 ARP 6
AR6 301h AR6 301h
Data Memory Data Memory
301h 02h 301h 02h
ACC 1 04h ACC 1 02h
CC
In the first example, C is originally zeroed, presumably from the result of a pre-
vious subtract instruction that performed a borrow. The effective operation per-
formed was 6 – 6 – (0–) = –1, generating another borrow (resetting carry) in
the process. In the second example, no borrow was previously generated (C
= 1), and the result from the subtract instruction does not generate a borrow.