TSK3000A Core Instruction - SUB

Frozen Content

Modified by Admin on Sep 13, 2017

Instruction:

Subtract Word

Assembler Format:

sub rC, rA, rB

Example:

sub $3, $4, $5

Description:

Subtracts the contents of GPR rB from GPR rA and puts the result in GPR rC.

Operation:

rC <-- rA - rB

Instruction Type:

R-Type

Instruction Fields:

rA = Register index of operand A

rB = Register index of operand B

rC = Register index of destination

Encoding:

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	0	0	0	0	rA					rB					rC					0	0	0	0	0	1	0	0	0	1	1

Latency: 1

Notes:

The following code example illustrates how overflow detection can be handled, in software, when subtracting two signed operands:

sub rC, rA, rB
xor rD, rA, rB -----compare sign of operand rA with operand rB
xor rE, rA, rC -----compare sign of operand rA and the difference
and rD, rD, rE -----bitwise logically AND the comparison values
slt rD, rD, $0 -----if result less than '0', flag overflow

rD will be set to '1' if an overflow occurred, otherwise it will be set to '0'