In computer engineering, a register–memory architecture is an instruction set architecture that allows operations to be performed on (or from) memory, as well as registers.^[1] If the architecture allows all operands to be in memory or in registers, or in combinations, it is called a "register plus memory" architecture.^[1]

In a register–memory approach one of the operands for operations such as the ADD operation may be in memory, while the other is in a register. This differs from a load–store architecture (used by RISC designs such as MIPS) in which both operands for an ADD operation must be in registers before the ADD.^[1]

Example of Load-Store (RISC)

;Add mem1 to mem2
mov r1, [mem1]       ; Load first value into register
mov r2, [mem2]       ; Load second value into register
add r2, r1, r2       ; r2 = r1 + r2
mov [mem2],r2        ; Store result

Example of register-memory (CISC)

Some register-memory machines cannot write ALU results to memory, only their registers. This is like an accumulator machine with multiple accumulators:

;Add mem1 to mem2, if add-register-to-memory instructions are not supported
mov r1, [mem1]       ; Load first value into register
add r1, [mem2]       ; Add second value into register
mov [mem2], r1       ; Store result

Some register-memory machines are able to write ALU results directly to memory, saving an instruction:

;Add mem1 to mem2, if add-register-to-memory instructions are supported
mov r1, [mem1]       ; Load first value into register
add [mem2],r1        ; Add it to second value

Example of register plus memory (CISC)

;Add mem1 to mem2
add [mem2],[mem1]       ; Add first to second value

An example of register-memory architecture that can write ALU results to memory is the Intel x86.^[1]

Examples of register plus memory architecture are:

• IBM System/360 and its successors, which support memory-to-memory fixed-point decimal arithmetic operations, but not binary integer or floating-point arithmetic operations;^[2][3][4]
• PDP-11, which supports memory (or register) source and destination operands for most two-operand integer operations;^[5]
• VAX, which supports memory or (or register) source and destination operands for binary integer and floating-point arithmetic;^[6]
• Texas Instruments MSP430 is a rare microcontroller that supports memory (or register) source and destination operands for all 24 of its two-operand operations.^[7]
• Motorola 68000 series can move data memory-to-memory with nearly all addressing modes. All other integer arithmetic/logic operations require at least one parameter to reside in a register, making it primarily a register-memory machine.^[8]

• Load–store architecture
• Addressing mode