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MicroBlaze Processor Reference Guide www.xilinx.com 127
UG081 (v6.0) June 1, 2006 1-800-255-7778
Instructions
R
rsub
Arithmetic Reverse Subtract
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 a one for
the mnemonic rsubk. Bit 4 of the instruction (labeled as C in the figure) is set to a one for
the mnemonic rsubc. Both bits are set to a 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 a 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 flagdoes not affect the execution ofthe instruction (providing anormal
subtraction).
Pseudocode
if C = 0 then
(rD) ← (rB) + (rA) + 1
else
(rD) ← (rB) + (rA) + MSR[C]
if K = 0 then
MSR[C] ← CarryOut
Registers Altered
• rD
• MSR[C]
Latency
1 cycle
Notes
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.
rsub
rD, rA, rB Subtract
rsubc
rD, rA, rB Subtract with Carry
rsubk
rD, rA, rB Subtract and Keep Carry
rsubkc
rD, rA, rB Subtract with Carry and Keep Carry
0 0 0 K C 1 rD rA rB 0 0 0 0 0 0 0 0 0 0 0
0 6 11 16 21 31