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220 Part 2½ Regions of Computer Space Section 1 ½ Microprogram-Based Processors

Out of sixteen Am2910 instructions, only 4 are used in this example: Conditional Jump Subroutine (CJS, #01), Conditional Jump (CJP, #03), Conditional Return from Subroutine (CRTN, #12), and Continue (CONT, #16). Therefore, it is theoretically possible to use only 2 bits of information to specify these four actions.

Table 1 The Microword Format and Required Bits per Field
Bits per field  Normal Minimum Actual (ISP)
Micro sequencer control
Microinstruction  4 2 6
Next microaddress 12 7 9
Condition code select (6) 6 6
ALU control
ALU instruction 
Source  3
Function 3 3 3
Destination  3
RAM A port select  3
RAM B port select  4 3
Direct input select (2) 3
Constant mask select (3)  3
Miscellaneous control signals
Control signal select  (4)  6
Total  48 39 48

Condition Code Input Selection

There is only one condition code (CC) input for the Am2910. The status conditions have to be multiplexed into this input. The assignments for the multiplexer input lines can be found in the ISP description in Appendix 1 (ISPS procedure Condition .Code). Five bits are used to select one out of 32 different input signals. The sixth bit in this field is used to select between the original signal and the complement of the signal. In this manner, the micromachine can branch when the signal is either high or low. When an unconditional microprogram branch is required, a logic 0 can be selected for the CC input.

Each bit from the Instruction Register (IR, 5 bits) or from the Memory Buffer Register (MBR, 12 bits) can be selected individually. This capability is used for the basic PDP-8 instruction decode, effective address calculation, and the Group 7 microinstruction decode. Random combinational logic is used to generate a single skip enable signal for the portion of the microprogram that decodes the PDP-8 skip conditions. Interrupt requests are also handled by using combinational logic in a similar manner.

ALU Operations and the Link Bit

Three Am2901 ALU chips are cascaded to form the PDP-8 ALU section. The ALU requires a 9-bit opcode: source, function, and destination. Six bits are used to encode the A port (3 bits) and B port (3 bits) select, since only a subset of the sixteen ALU RAM registers is used in this implementation.

The PDP-8 Link bit is constructed from random logic con trolled by a set of signals. For economic reasons, random logic is used rather than adding another Am2901 chip. The Link bit does not correspond to any Am2901 function, and its control would

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