shamt

Shamt, short for shift amount, is a field used in some CPU instruction encodings to specify the number of bit positions by which a value should be shifted. In the MIPS R-type instruction format, shamt is a five-bit field that provides the immediate shift distance for fixed-shift operations.

In MIPS, the R-type instruction format is opcode (6 bits), rs (5 bits), rt (5 bits), rd (5 bits), shamt (5 bits), and funct (6 bits). For the shift instructions sll (shift left logical), srl (shift right logical), and sra (shift right arithmetic), the rs field is typically unused (often zero), the value to shift is in rt, the destination is rd, and shamt contains the shift amount, ranging from 0 to 31. For example, sll rd, rt, shamt shifts rt left by shamt positions and writes the result to rd.

There are also variable-shift instructions, such as sllv, srlv, and srav, where the shift amount comes from a register rather than the shamt field. In these instructions, the syntax is typically sllv rd, rt, rs, and the amount is taken from the rs register (usually using only its low 5 bits).

Beyond MIPS, the concept of a dedicated shift amount field exists in other instruction set architectures, though the exact field name and encoding can differ. The general purpose remains to encode how many bit positions a value should be shifted, either as an immediate value or sourced from a register.