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(In the examples that follow, a, b, and c are (direct or calculated) addresses referring to memory cells, while reg1 and so on refer to machine registers.) C = A+B 0-operand (zero-address machines), so called stack machines: All arithmetic operations take place using the top one or two positions on the stack: [9] push a, push b, add, pop c.
The number of operands is one of the factors that may give an indication about the performance of the instruction set. A three-operand architecture (2-in, 1-out) will allow A := B + C to be computed in one instruction ADD B, C, A A two-operand architecture (1-in, 1-in-and-out) will allow A := A + B to be computed in one instruction ADD B, A
Instructions included an address for the operand. For instance, an ADD address instruction would cause the CPU to retrieve the number in memory found at that address and then add it to the value already in the accumulator. This very simple example ISA has a "one-address format" because each instruction includes the address of the data. [4]
The logical address space (that is, the address space available at any moment without changing the memory mapping table) remains limited to 16 bits. Some models, beginning with the PDP-11/45, can be set to use 32K words (64 KB) as the "instruction space" for program code and a separate 32K words of "data space".
This sort of design, which maximized performance by carefully tuning the instruction set for minimal size, was common until the 1980s. An example of this approach is the MOS 6502 , which had only a single register, in which case it is referred to as the accumulator , and a special "zero page" addressing mode that treated the first 256 bytes of ...
A standard instruction set, including fixed-point binary arithmetic and logical instructions, present on all System/360 models (except the Model 20, see below). A commercial instruction set, adding decimal arithmetic instructions, is optional on some models, as is a scientific instruction set, which adds
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A specification for a 64-bit superscalar design, "Rocket", is available for download. It is implemented in the European Processor Initiative processor. The ARM architecture currently in use by cloud providers for servers. One example is the AWS Graviton series processor used for various services on the AWS platform. [66]