Dale - Computer Science Illuminated 223

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196 Chapter 7 Low-Level Programming Languages Because the register specifier is 1, the contents of word 001F (bytes 001F and 0020) would be loaded into register X. Otherwise, this instruction is identical to the one before it. 11101 Store register to operand This instruction stores the contents of either the A register or the X register into the location specified in the operand, which is either the operand itself or the place named in the operand. This instruction stores the contents of the A register into the word begin- ning at location 000A. It is invalid to have an addressing mode of imme- diate with a store opcode; that is, we cannot try to store the contents of a register into the operand specifier. 11100 Add operand to register Like the Load operation, the Add opera- tion uses both the register specifier and the addressing mode specifier, giving alternative interpretations. The four alternatives for this instruction
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Unformatted text preview: are shown below with the explanation following each instruction. The contents of the second and third bytes of the instruction (the operand specifier) are added to the contents of the A register (20A in hex). Thus we have shaded the operand specifier to show that it is data. The contents of the second and third bytes of the instruction (the operand specifier) are added to the contents of the X register. Instruction specifier Operand specifier 1 1 0 0 0 0 0 0 1 0 0 0 0 0 1 0 1 0 1 Instruction specifier Operand specifier 1 1 1 0 0 0 0 0 0 1 0 0 0 0 0 1 0 1 0 Instruction specifier Operand specifier 1 1 0 0 0 0 0 0 1 0 0 0 0 0 1 0 1 0 Instruction specifier Operand specifier 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1...
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This note was uploaded on 01/13/2011 for the course CSE 1550 taught by Professor Marianakant during the Fall '10 term at York University.

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