Complete Computer Description

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The final flowchart of the instruction cycle, including the interrupt cycle for the basic computer, is shown in Fig. 5-15. The interrupt flip-flop R may be set at any time during the indirect or execute phases. Control returns to timing signal T0 after SC is cleared to 0. If R = 1, the computer goes through an interrupt cycle. If R = 0, the computer goes through an instruction cycle. If the instruction is one of the memory-reference instructions, the computer first checks if there is an indirect address and then continues to execute the decoded instruction according to the flowchart of Fig. 5-1 L If the instruction is one of the register-reference instructions, it is executed with one of the microoperations listed in Table 5-3. If it is an input-output instruction, it is executed with one of the microoperations listed in Table 5-5.

Instead of using a flowchart, we can describe the operation of the computer with a list of register transfer statements. This is done by accumulating all the control functions and microoperations in one table. The entries in the table are taken from Figs. 5-11 and 5-15, and Tables 5-3 and 5-5.

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The control functions and microoperations for the entire computer are summarized in Table 5-6. The register transfer statements in this table describe in a concise form the internal organization of the basic computer. They also give all the information necessary for the design of the logic circuits of the computer. The control functions and conditional control statements listed in the table formulate the Boolean functions for the gates in the control unit. The list of microoperations specifies the type of control inputs needed for the registers and memory. A register transfer language is useful not only for describing the internal organization of a digital system but also for specifying the logic circuits needed for its design.

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