Xilinx EDK 8.2i manual Arithmetic Reverse Subtract Immediate Rsubi, Rsubic, Rsubikc

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Chapter 4: MicroBlaze Instruction Set Architecture

rsubi

Arithmetic Reverse Subtract Immediate

rsubi	rD, rA, IMM	Subtract Immediate
rsubic	rD, rA, IMM	Subtract Immediate with Carry
rsubik	rD, rA, IMM	Subtract Immediate and Keep Carry
rsubikc	rD, rA, IMM	Subtract Immediate with Carry and Keep Carry

0 0 1 K C 1

rD

rA

IMM

0 6 11 16 31

Description

The contents of register rA is subtracted from the value of IMM, sign-extended to 32 bits, 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 rsubik. Bit 4 of the instruction (labeled as C in the figure) is set to a one for the mnemonic rsubic. Both bits are set to a one for the mnemonic rsubikc.

When an rsubi instruction has bit 3 set (rsubik, rsubikc), the carry flag will Keep its previous value regardless of the outcome of the execution of the instruction. If bit 3 is cleared (rsubi, rsubic), then the carry flag will be affected by the execution of the instruction. When bit 4 of the instruction is set to a one (rsubic, rsubikc), the content of the carry flag (MSR[C]) affects the execution of the instruction. When bit 4 is cleared (rsubi, rsubik), the content of the carry flag does not affect the execution of the instruction (providing a normal subtraction).

Pseudocode

if C = 0 then

(rD) ← sext(IMM) + (rA) + 1 else

(rD) ← sext(IMM) + (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.

By default, Type B Instructions will take the 16-bit IMM field value and sign extend it to 32 bits to use as the immediate operand. This behavior can be overridden by preceding the Type B instruction with an imm instruction. See the imm instruction for details on using 32-bit immediate values.

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