Xilinx MicroBlaze Reference Manual page 273

rsub Arithmetic Reverse Subtract
rD, rA, rB
rsub
rsubc rD, rA, rB
rD, rA, rB
rsubk
rsubkc rD, rA, rB
0 0 0 K C 1
0
6
Description
The contents of register rA is subtracted from the contents of register rB and the result is placed into
register rD. Bit 3 of the instruction (labeled as K in the figure) is set to one for the mnemonic rsubk.
Bit 4 of the instruction (labeled as C in the figure) is set to one for the mnemonic rsubc. Both bits are
set to one for the mnemonic rsubkc.
When an rsub instruction has bit 3 set (rsubk, rsubkc), the carry flag will Keep its previous value
regardless of the outcome of the execution of the instruction. If bit 3 is cleared (rsub, rsubc), then the
carry flag will be affected by the execution of the instruction.
When bit 4 of the instruction is set to one (rsubc, rsubkc), the content of the carry flag (MSR[C]) affects
the execution of the instruction. When bit 4 is cleared (rsub, rsubk), the content of the carry flag does
not affect the execution of the instruction (providing a normal subtraction).
Pseudocode
if C = 0 then
←
(rD) (rB) +
else
←
(rD) (rB) +
if K = 0 then
←
MSR[C] CarryOut
Registers Altered
• rD
• MSR[C]
Latency
• 1 cycle
Note
In subtractions, Carry = (Borrow). When the Carry is set by a subtraction, it means that there is no
Borrow, and when the Carry is cleared, it means that there is a Borrow.
MicroBlaze Processor Reference Guide
UG984 (v2018.2) June 21, 2018
Subtract
Subtract with Carry
Subtract and Keep Carry
Subtract with Carry and Keep Carry
rD rA
11
(rA) + 1
(rA)
+ MSR[C]
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Chapter 5: MicroBlaze Instruction Set Architecture
rB 0 0 0 0 0 0 0 0 0 0 0
16 21
31
274
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