128 www.xilinx.com MicroBlaze Processor Reference Guide
1-800-255-7778 UG081 (v6.0) June 1, 2006
Chapter 4: MicroBlaze Instruction Set Architecture
R
rsubi
Arithmetic Reverse Subtract Immediate
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 setto aone forthe mnemonicrsubic. Both bits are setto 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 theinstruction 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 BInstructionswill take the16-bit IMM fieldvalue and sign extendit 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.
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