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Page 5 Table 1.1 Notation for microprograms and state transitions. R1¬ R2 Copy contents of R2 into R1. SR ¬ Mem [MAR] or Memory read Memory read operation: the contents of the address in memory address register (MAR) fetched into the memory output storage register (SR). Mem [MAR] ¬ SR or Memory write Memory write operation: data in the storage register (SR) copied into the location specified by the address in the memory address register (MAR). PC ¬* + 1 or PC ¬ PC+1 The current value * in the program counter is incremented by 1. MAR ¬ [RX] + [RB] + D Contents of register specified by RX plus contents of register specified by RB plus D are transferred to the MAR. [RX] indicates that RX is a register address. The instruction (Figure 1.3) is a vector of bits partitioned into fields. These fields identify properties of, or actions to be taken by, an instruction, including: 1. Format. 2. Object address (identifiers)the "A," "B," and "C" fields in Figure 1.3. 3. Operationthe "OP" in Figure 1.3. 4. Sequence control. Some fields (or actions) may be missing from an instruction either because they are implied or are unnecessary. Notation for describing actions within instruction execution is shown in Table 1.1. 1.2 The Machine: Interpretation and Microprogramming Management of the interpretation process is the responsibility of the decoder (a part of the implementation mechanism). The process of interpreting or executing an instruction begins with the decoding of the opcode field from the instruction. The decoder activates storage and registers for a series of state transitions that correspond to the action of the opcode. The image machine storage consists of registers and memory, which are both explicitly stated in the instruction and implicitly defined by the instruction. Explicit registers include: 1. General purpose registers (GPR). 2. Accumulators (ACC). 3. Address registers (index or base registers).
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