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Some instructions give one or both operands implicitly, such as by being stored on top of the stack or in an implicit register. If some of the operands are given implicitly, fewer operands need be specified in the instruction. When a "destination operand" explicitly specifies the destination, an additional operand must be supplied.
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
Most instructions have one or more opcode fields that specify the basic instruction type (such as arithmetic, logical, jump, etc.), the operation (such as add or compare), and other fields that may give the type of the operand(s), the addressing mode(s), the addressing offset(s) or index, or the operand value itself (such constant operands ...
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.
A specification exception PoOps: 80 is recognized when an instruction has a length or register field with values not permitted by the operation, or when it has an operand address that does not satisfy the alignment requirements of the opcode, e.g., a LH instruction with an odd operand address on a machine without the byte alignment feature.
Instead of using an operand from memory, the value of the operand is held within the instruction itself. On the DEC VAX machine, the literal operand sizes could be 6, 8, 16, or 32 bits long. Andrew Tanenbaum showed that 98% of all the constants in a program would fit in 13 bits (see RISC design philosophy ).
This leaves 14 bits, the first of which indicates whether the following 13 contain an immediate value or uses only five of them to indicate a register for the second operand. [22] A more complex example is the MIPS encoding, which used only 6 bits for the opcode, followed by two 5-bit registers.
In the first example, the operand 61h is a valid hexadecimal numeric constant and is not a valid register name, so only the B0 instruction can be applicable. In the second example, the operand AH is a valid register name and not a valid numeric constant (hexadecimal, decimal, octal, or binary), so only the 88 instruction can be applicable.