One of the key features used to categorize a microprocessor is whether it supports reduced instruction set computing (RISC—pronounced “risk”) or complex instruction set computing
(CISC—pronounced “sisk”). The distinction is how complex individual instructions are and how many permutations exist for the same basic instruction. In practical terms, this distinction directly relate to the complexity of a microprocessor’s instruction decoding logic; a more complex instruction set requires more complex decoding logic. Some engineers believe that a microprocessor should exe-cute simple instructions at a high rate—perhaps one instruction per cycle. Others believe that a microprocessor should execute more complex instructions at a lower rate. Operand types add complexity to an instruction set when a single general operation such as addition can be invoked with many different addressing modes. Motorola’s CISC 68000 contains a basic addition instruction, among other addition operations, that can be decoded in many different ways according to the specified addressing mode. Table 7.1 shows the format of the basic ADD / ADDA / ADDX instruction word.
ADD is used for operations primarily on data registers. ADDA is used for operations primarily on address registers. ADDX is used for special addition operations that incorporate the ALU extended carry bit, X, into the sum. The instruction word references Register1 directly and
an effective address (EA) that can represent another register or various types of indirect and indexed addressing modes.
(CISC—pronounced “sisk”). The distinction is how complex individual instructions are and how many permutations exist for the same basic instruction. In practical terms, this distinction directly relate to the complexity of a microprocessor’s instruction decoding logic; a more complex instruction set requires more complex decoding logic. Some engineers believe that a microprocessor should exe-cute simple instructions at a high rate—perhaps one instruction per cycle. Others believe that a microprocessor should execute more complex instructions at a lower rate. Operand types add complexity to an instruction set when a single general operation such as addition can be invoked with many different addressing modes. Motorola’s CISC 68000 contains a basic addition instruction, among other addition operations, that can be decoded in many different ways according to the specified addressing mode. Table 7.1 shows the format of the basic ADD / ADDA / ADDX instruction word.
ADD is used for operations primarily on data registers. ADDA is used for operations primarily on address registers. ADDX is used for special addition operations that incorporate the ALU extended carry bit, X, into the sum. The instruction word references Register1 directly and
an effective address (EA) that can represent another register or various types of indirect and indexed addressing modes.
