In addition to these 32 general-purpose registers, the CPU has a few special-purpose registers: Some variants of the MUL operation are limited to eight registers, R16 through R23. Instructions which allow an immediate value are limited to registers R16–R31 (8-bit operations) or to register pairs R25:R24–R31:R30 (16-bit operations ADIW and SBIW). Y and Z also support a six-bit positive displacement. Postincrement and predecrement addressing modes are supported on all three. The last three register pairs are used as pointer registers for memory addressing. The lower-numbered register of the pair holds the least significant bits and must be even-numbered. All arithmetic and logic operations operate on those registers only load and store instructions access RAM.Ī limited number of instructions operate on 16-bit register pairs. There are 32 general-purpose 8-bit registers, R0–R31.
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