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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]
Unlike 2-operand or 1-operand, this leaves all three values a, b, and c in registers available for further reuse. [11] more operands—some CISC machines permit a variety of addressing modes that allow more than 3 operands (registers or memory accesses), such as the VAX "POLY" polynomial evaluation instruction.
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
In computing, an operand is the part of a computer instruction which specifies what data is to be manipulated or operated on, while at the same time representing the data itself. [5] A computer instruction describes an operation such as add or multiply X, while the operand (or operands, as there can be more than one) specify on which X to ...
Double words are used by the MUL, DIV, and ASHC instructions. Other 32-bit data are supported as extensions to the basic architecture: floating point in the FPU Instruction Set or long data in the Commercial Instruction Set are stored in more than one format, including an unusual middle-endian format [2] [3] sometimes referred to as "PDP-endian."
In the following examples, computed values are in bold, while Register numbers are not. For example, to write the value 3 to register 1, (which already contains a 6), and then add 7 to register 1 and store the result in register 2, i.e.: i0: R1 = 6 i1: R1 = 3 i2: R2 = R1 + 7 = 10. Following execution, register 2 should contain the value 10.
This technique is an efficient way to store a number of Boolean values using as little memory as possible. For example, 0110 (decimal 6) can be considered a set of four flags numbered from right to left, where the first and fourth flags are clear (0), and the second and third flags are set (1).
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 ...