Lecture 11: Computer Instructions

#InstructionFormat #MemoryReference #RegisterReference #Input/Output The basic computer has three instruction code formats, as shown in Fig. 3-1. Each format has 16 bits. The operation code (opcode) part of the instruction contains three bits and the meaning of the remaining 13 bits depends on the operation code encountered. A memory reference instruction uses 12 bits to specify an address and one bit to specify the addressing mode I. I is equal to 0 for direct address and to 1 for indirect address. The register-reference instructions are recognized by the operation code 111 with a 0 in the leftmost bit (bit 15) of the instruction. A register-reference instruction specifies an operation on or a test of the AC register. An operand from memory is not needed; therefore, the other 12 bits are used to specify the operation or test to be executed. An input-output instruction does not need a reference to memory and is recognized by the operation code 111 with a 1 in the leftmost bit of the instruction. The remaining 12 bits are used to specify the type of input-output operation or test performed. The type of instruction is recognized by the computer control from the four bits in positions 12 through 15 of the instruction. If the three op code bits in positions 12 though 14 are not equal to 111, the instruction is a memory-reference type and the bit in position 15 is taken as the addressing mode I. If the 3-bit op code is equal to 111, control then inspects the bit in position 15. If this bit is 0, the instruction is a register-reference type. If the bit is 1, the instruction is an input-output type. Note that the bit in position 15 of the instruction code is designated by the symbol I but is not used as a mode bit when the operation code is equal to 111. Only three bits of the instruction are used for the operation code. It may seem that the computer is restricted to a maximum of eight distinct operations. However, since register reference and input-output instructions use the remaining 12 bits as part of the operation code, the total number of instruction chosen for the basic computer is equal to 25.