Xilinx EDK 8.2i manual Immediate ImmIMM

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Chapter 4: MicroBlaze Instruction Set Architecture

imm

Immediate

immIMM

1 0 1 1 0 0

0 0 0 0 0

0 0 0 0 0

IMM

0 6 11 16 31

Description

The instruction imm loads the IMM value into a temporary register. It also locks this value so it can be used by the following instruction and form a 32-bit immediate value.

The instruction imm is used in conjunction with Type B instructions. Since Type B instructions have only a 16-bit immediate value field, a 32-bit immediate value cannot be used directly. However, 32-bit immediate values can be used in MicroBlaze. 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. The imm instruction locks the 16-bit IMM value temporarily for the next instruction. A Type B instruction that immediately follows the imm instruction will then form a 32-bit immediate value from the 16-bit IMM value of the imm instruction (upper 16 bits) and its own 16-bit immediate value field (lower 16 bits). If no Type B instruction follows the IMM instruction, the locked value gets unlocked and becomes useless.

Latency

1 cycle

Notes

The imm instruction and the Type B instruction following it are atomic, hence no interrupts are allowed between them.

The assembler provided by Xilinx automatically detects the need for imm instructions. When a 32-bit IMM value is specified in a Type B instruction, the assembler converts the IMM value to a 16-bit one to assemble the instruction and inserts an imm instruction before it in the executable file.

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