Left Hand Side Concatenation
Instruction decoding in a microprocessor pipeline requires extracting functional fields like opcodes and immediate values from a fetched instruction word. Assigning slices of a wide bus to individual signals improves readability and prevents off-by-one bit-slicing errors throughout the rest of the decoding logic.
This module takes a 16-bit instruction word and splits it into two distinct fields. The most significant 4 bits represent the operation code, and the least significant 12 bits represent the data payload. Instead of assigning each output individually using bit slices on the right side of the assignment, the design assigns both outputs simultaneously using left-hand side concatenation.
This circuit is purely combinational. Output values must reflect the current input values without any clock or reset dependency. For example, if the input inst is 16'hA123, the module isolates the upper 4 bits 4'hA for the opcode and the lower 12 bits 12'h123 for the payload.
| Signal | Direction | Width | Description | |--------|-----------|-------|-------------| | inst | input | 16 | The full instruction word to be decoded | | opcode | output | 4 | The most significant 4 bits of the instruction | | payload | output | 12 | The least significant 12 bits of the instruction |
Constraints
- The circuit is purely combinational; no registers or latches are permitted
- The
opcodeoutput must continuously reflectinst[15:12] - The
payloadoutput must continuously reflectinst[11:0]
Topics
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