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Section 2½ Microcomputers 611


Table 2 Microcomputers Based on Minicomputers

Intersil 6100

LSI-11

LSI-11/23

TI TMS9900

Micro Nova MN601

Fairchild 9400

Technology

CMOS

NMOS

NMOS

NMOS

NMOS

I2L

Number of chips

1

4

2

1

4

1

Number of pins per package

40

40

40†

64

40

40

Cycle time (m s)

5

0.4

0.29

0.25

?

0.1

Register move time (m s)

N.A.

3.5

1.7

4.7

2.4

1.5

Data-path width (bits)

12

8

16

16

16

4

Maximum memory size (bytes)

32K

64K

256K

64K

32K

64K

Microcode size

. . .

1,024 ´ 22

522 ´ 25

. . .

. . .

. . .

Register file size

1

26

16

16 (In memory)

4

4

Stack size

. . .

In RAM

In RAM

. . .

In RAM

. . .

Instruction set emulated

PDP-8

PDP-11/40

PDP-11/34

TI-990

Data General Nova

Data General Nova

Year introduced

1975

1975

1979

1976

1976

1978


† Two chips per 40-pin chip carrier.

(e.g., register file or memory buffer registers). See Part 2, Sec. 1. Microprogram sequencing is provided by incrementing a 4-bit microprogram counter, which is concatenated to an 8-bit ROM address register. Sequencing can be modified by a short jump (jamming a 4-bit microword subfield into the microprogram counter) or a long jump (by having 4 bits specify one of sixteen 12-bit addresses stored in an auxiliary ROM).

The 8086 has a two-stage pipeline composed of instruction fetch and execution. A 6-byte buffer allows prefetching of instructions during long-execution-time instructions and supplying instructions with no memory latency following short-execution-time instructions. The instruction execution stage of the pipeline also allows for partial overlap of current instruction execution with next instruction decoding.

Microcomputers Based on Minicomputers

Rather than evolve new instruction sets, some of the simpler existing instruction sets with large software bases could be implemented. Table 2 is a small sample of this growing class of microcomputer systems which are based on minicomputers. It is interesting to note that the PDP-8 was a very early, if not the first, minicomputer, and it was also implemented early as a one-chip processor. Chapters 49 and 31 describe microcomputer implementations of an HP 2116-like ISP. The HP 2116 is a 16-bit minicomputer with a close kinship to the PDP-8 ISP.

References

Adams [1978]; Holt [1974]; McKevitt [1979].

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