# 74181

The 74S181 4-bit ALU bitslice resting on a page from the datasheet.

The 74181 is a bit slice arithmetic logic unit (ALU), implemented as a 7400 series TTL integrated circuit. The first complete ALU on a single chip,[1] it was used as the arithmetic/logic core in the CPUs of many historically significant minicomputers and other devices.

The 74181 represents an evolutionary step between the CPUs of the 1960s, which were constructed using discrete logic gates, and today's single-chip CPUs or microprocessors. Although no longer used in commercial products, the 74181 is still referenced in computer organization textbooks and technical papers. It is also sometimes used in 'hands-on' college courses, to train future computer architects.

## Specifications

The combinational logic circuitry of the 74181 integrated circuit

The 74181 is a 7400 series medium-scale integration (MSI) TTL integrated circuit, containing the equivalent of 75 logic gates[2] and most commonly packaged as a 24-pin DIP. The 4-bit wide ALU can perform all the traditional add / subtract / decrement operations with or without carry, as well as AND / NAND, OR / NOR, XOR, and shift. Many variations of these basic functions are available, for a total of 16 arithmetic and 16 logical operations on two four-bit words. Multiply and divide functions are not provided but can be performed in multiple steps using the shift and add or subtract functions. Shift is not an explicit function but can be derived from several available functions including (A+B) plus A, A plus AB[clarification needed].

The 74181 performs these operations on two four-bit operands generating a four-bit result with carry in 22 nanoseconds (45 MHz). The 74S181 performs the same operations in 11 nanoseconds (90 MHz), while the 74F181 performs the operations in 7 nanoseconds (143 MHz) (typical).

Multiple 'slices' can be combined for arbitrarily large word sizes. For example, sixteen 74S181s and five 74S182 look ahead carry generators can be combined to perform the same operations on 64-bit operands in 28 nanoseconds (36 MHz). Although overshadowed by the performance of today's multi-gigahertz 64-bit microprocessors, this was quite impressive when compared to the sub megahertz clock speeds of the early four and eight bit microprocessors.

### Function Table

 Selection ACTIVE LOW INPUTS & OUTPUTS ACTIVE HIGH INPUTS & OUTPUTS S3 S2 S1 S0 Logic (M = 1) Arithmetic (M = 0) (Cn = 0) Logic (M = 1) Arithmetic (M = 0) (Cn = 1) 0 0 0 0 ${\displaystyle {\overline {A}}}$ ${\displaystyle A-1}$ ${\displaystyle {\overline {A}}}$ ${\displaystyle {A}}$ 0 0 0 1 ${\displaystyle {\overline {AB}}}$ ${\displaystyle AB-1}$ ${\displaystyle {\overline {A+B}}}$ ${\displaystyle {A+B}}$ 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 1 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 ${\displaystyle A}$ ${\displaystyle A}$ ${\displaystyle A}$ ${\displaystyle {A}}$ minus ${\displaystyle 1}$

## Significance

Although the 74181 is only an ALU and not a complete microprocessor, it greatly simplified the development and manufacture of computers and other devices that required high speed computation during the late 1960s through the early 1980s, and is still referenced as a "classic" ALU design.[3]

Prior to the introduction of the 74181, computer CPUs occupied multiple circuit boards and even very simple computers could fill multiple cabinets. The 74181 allowed an entire CPU and in some cases, an entire computer to be constructed on a single large printed circuit board. The 74181 occupies a historically significant stage between older CPUs based on discrete logic functions spread over multiple circuit boards and modern microprocessors that incorporate all CPU functions in a single component. The 74181 was used in various minicomputers and other devices beginning in the 1970s, but as microprocessors became more powerful the practice of building a CPU from discrete components fell out of favor and the 74181 was not used in any new designs.

## Today

CPU designs based on the 74181 are not commercially viable today due to the comparatively low price and high performance of microprocessors. However, the 74181 is still of interest in the teaching of computer organization and CPU design because it provides opportunities for hands-on design and experimentation that are rarely available to students.[4]

## Computers

Many computer CPUs and subsystems were based on the 74181, including several historically significant models.

## References

1. ^ Daniel P. Sieworek; C. Gordon Bell; Allen Newell. "Chapter 6: Structure". Computer Structures: Principles and Examples (PDF). p. 63. The earliest and most famous chip, the 74181 arithmetic logic unit (ALU), provided up to 32 functions of two 4-bit variables.
2. ^ Miles Murdocca, Apostolos Gerasoulis, and Saul Levy. "Novel Optical Computer Architecture Utilizing Reconfigurable Interconnects". 1991. p. 23. quote: "Logic diagram for the 74181 ... There are 63 logic gates."
3. ^ Kestrel: Design of an 8-bit SIMD parallel processor (PDF). Proc. 17th Conf. on Advanced Research in VLSI. September 15–17, 1997. p. 11.
4. ^ Bradford J. Rodriguez. A Minimal TTL Processor for Architecture Exploration. Proceedings of the 1994 ACM Symposium on Applied Computing. The study of computer architecture is often an abstract, paper exercise. Students cannot probe the inner workings of a single-chip microprocessor, and few discrete-logic machines are open to student inspection.
5. ^ History of the 74181 in commercial minicomputers
6. ^ C. Gordon Bell; J. Craig Mudge; John E. McNamara (August 1979). Computer Engineering: A DEC View of Hardware Systems Design. Digital Press. p. 335,336. ISBN 0-932376-00-2.
7. ^ a b Bob Supnik (August 31, 2004). "Simulators: Virtual Machines of the Past (and Future)". ACM Queue. 2 (5).
8. ^ "Computer History Museum - final demonstration of the Xerox 'Star' Computer". Archived from the original on 2007-11-15. Retrieved 2007-10-28.
9. ^
10. ^ "VAX-11/780, in Digital Computing Timeline, 1977". Digital Information Research Services, via Microsoft Research (research.microsoft.com). 30 April 1998. Retrieved 2007-11-02.
11. ^
12. ^ Early PDP-11 peripherals
13. ^ FPP-12 -- The PDP-12 Floating Point Unit
14. ^ Wang 2200 CPU Microarchitecture
15. ^ Wang 2200 Disk Channel Description
16. ^ Datapoint 2200 Drawing Package p36
17. ^ "ICL 1501 Intelligent Terminal". Allard's Computer Museum Groningen. Retrieved 23 April 2015.
18. ^ Star Castle Op and Maintenance. Cinematronics, Inc. 1980.