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386 Part 4 The instruction-set processor level: special-function processors

Section 4 ÷ Processors based on a programming language

Fig. 4. Simplified data flow of the IBM System/360 Model 30.

data flow registers and the Arithmetic-Logic-Unit (ALU), microprogram sequencing and branching control, and status hit-setting control. Microprogram words are stored in a Card Capacitor Read-Only Storage (CCROS). Fetching one microprogram word and executing it takes 750 nsec, the basic machine cycle.

Figure 4 shows in simplified form the data flow of the IBM System/360 (IBM 2030 CPU). It consists of a core storage with up to 65,536 8-bit bytes and a local storage (accessible by the microprogrammer but not explicitly by the 360 language programmer), a 16-bit storage address register (M, N), a set of 10 8-bit data registers (I, J, . . . , R), an arithmetic-logic-unit (ALU), connecting 8-bit wide buses (Z, A, B, M, N-bus), temporary registers (A, B), switches and gates.

Figure 5 shows the more important fields of a microprogram word. Only 47 bits are shown. Other fields contain various parity bits and special control bits. The field interpretation given in Fig. 5 is as for microprogram words in the second Read-Only Storage unit (Compatibility ROS) if the machine is equipped with the 1620 Compatibility Feature. The meaning of the microprogram word fields is explained in connection with Fig. 6 which shows the symbolic representation of a microprogram word together with an example as it appears on a microprogram documentation sheet.

The fields of the microprogram word can be grouped in five categories:

1 ALU control fields: CA, CF, CB, CC, CV, CD, CC

2 Storage control fields: CM, CU

3 Microprogram sequencing and branching fields: CN, CH, CL

4 Status bit setting field: CS
5 Constant field: CK

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