SOB Instruction

Purpose:

Decrement integer loop count and loop

Format:

opcode index.ml, displ.bb

Operation:

index <- index - 1;
SOBGEQ
if index >= 0 then: PC <- PC + sign-extended displ
SOBGTR
if index > 0 then: PC <- PC + sign-extended displ

Condition
Codes:

N <- index LSS 0;
Z <- index EQL 0;
V <- Integer Overflow;
C <- C;

Exceptions:

Integer Oveflow

Opcodes:

F4	SOBGEQ	Subtract One and Branch Greater Than or Equal
F5	SOBGTR	Subtract One and Branch Greater Than

Description:

One is subtracted from the index operand and the index operand is replaced by the result. On SOBGEQ, if the index operand is greater than or equal to 0, the branch is taken. On SOBGTR, if the index operand is greater than 0, the branch is taken. If the branch is taken, the sign-extended displacement operand is added to the PC and the PC is replaced by the result.

Notes:

1.: Integer overflow occurs if the index operand, before the subtraction, is the largest possible negative integer. On integer overflow, the index operand is replaced by the largest positive integer, and thus the branch is taken.
2.: The C-Bit is unaffected.