Bit-Slice Design: Controllers and ALUs

by Donnamaie E. White

Copyright © 1996, 2001, 2002 Donnamaie E. White



Table of Contents

1. Introduction

2. Simple Controllers

3. Adding Programming Support to the Controller

4. Refining the CCU

5. Evolution of the ALU

6. The ALU and Basic Arithmetic

7. Tying the System Together





Last Edit September 24, 1996; May 1, 1999; July 9, 2001

Each of the last six decades has seen a new technology come forward as the leading edge for that era. Table 1 provides a summary of this evolution by decade and integration level.

Table 1 - Integrated Circuit Evolution

Approx. Date Size Description
1950s gate level
A few transistors and other components combined to form an AND, OR or NOR gates
mid 1960s SSI
Small Scale
4 or more gates (about 2-10 gates); NAND, NOR, OR, AND, EXOR, NOT or INVERT; where a few gates were lumped together as a means of improving the design and the design process
early 1970s MSI
Medium Scale
20-100 gates up to 200 gates; registers, decoders, multiplexors, counters, adders, comparators, etc.
when more gates were packed together in a single chip for the same reasons. MSI allows more modular designs, speeding the design process when the blocks can be applied
late 1970s LSI
Large Scale
200-1000+ (several hundred gates). In the late 1970s arithmetic logic units (ALUs) with on-board registers, microprogrammable sequencers and interrupt controllers in a bit-slice format became available. Memory chips (ROM, PROM, RAM) in increasing sizes became readily available. LSI culminated in the one-chip microprocessors.
1980s VLSI , VHLSI
Very Large Scale
(Very High-Speed
Large Scale
700 gates and up or 20,000 gates and higher; CPUs (central processor unit), complex functions; where larger functional blocks and their related circuitry could be brought together in lower power, faster chips. LSI and VLSI further increase the modular block size, reducing design time, space, and power considerations and increasing reliability as connections are moved inside the components. Many LSI and VLSI blocks are designed by their manufacturers and referred to as fixed-instruction-set (FIS) modules.
1980s ASIC up to 30,000 gates; multiple functions
early 1990s ASIC up to 100,000 gates and increasing with speeds at 1.4GHz and higher
1980-1990s ASSP application specific standard product
1990s EPAC Electrically Programmable Analog Circuit; The development of analog circuit arrays
1990s DSM DSM and VDSM, Deep Submicron with chips approaching 1 million gates
late 1990s Standard Cell Standard cell designs meet or beat ASICs in die size and speed of design thanks to the evolution of design tools
1990s capture Design methodology went from schematic capture to netlist driven (Verilog, VHDL) and designs became synthesized

Each technology change has led to a period where those designers who are state-of-the-art orientated, those who readily delve into new developments, accept and begin to use the newest devices in designs. For successful technologies, this is followed by the intense application and development phase where the high demand for engineers who can design with the devices typically exceeds the supply of those engineers.

Bit-Slice Design (LSI)

For any given design, if the architecture of the fixed LSI and VLSI blocks suit the application then the design time is considerably shortened. When a one-chip microprocessor is not quite suitable, microprogrammable architectures can often provide sufficient customization.

Microprogrammable architectures, such as bit-slice, allow a closer control over the architecture but not total control. The basic building blocks are still designed by the chip manufacturer for generic applications. Bit-slice architectures include interruptable sequencers and 32-bit ALUs.

The customization of the bit-slice modules to an application is done through customer-designed module interconnection, the implemented commands and their sequences. The commands or instruction set is called the micro-program for the design.


The 1980s saw the acceptance of ASICs ( application specific integrated circuits), VLSI devices large enough to allow designers to implement architectures that were suited to solving the design problem rather than forcing one architecture to solve everything. It was the natural extension to the bit-slice architectures, where some control of architecture was possible through microprogramming but where the basic building blocks were fixed designs.

Not far behind the ASIC and ASIC developments, multimedia and design integration saw a need to incorporate analog functions into digital systems. For years the trend had been away from analog design as a chosen career and now there was a shortage of design engineers. First came massive re-training of internal staff as companies struggled to cope. Then came the creation of Electrically Programmable Analog Circuit (EPAC) and related devices.

Application-specific solutions also includes the standard product mix where the market for a device is so large tht product are developed specific to a mass application. PCI controllers is an example where one interface controller is targeted to handle the interface for many devices and device types, the control problem tailored to the device via programming.

The application-specific customization of the design solution allows the designer to have the creative power of a gate-level breadboard design while keeping the production advantages of VLSI.


Once upon a time....

Over the years, there has been an evolution of the universal building blocks used by logic circuit designers. In the mid-1960s, there were SSI gates; NAND, NOR, EXOR, and NOT or INVERT. In the early 1970s, MSI blocks, registers, decoders, multiplexers, and other blocks made their appearances. In the late 1970s, ALUs (arithmetic logic units) with on-board scratchpad registers, interrupt controllers, microprogram sequencers, ROMs/PROMs, and other LSI devices up to and including a complete one-chip microprocessor (control, ALU and registers) became readily available. (And from this the PC was born.)

SSI (small scale integration) is defined here to include chips containing approximately 2-10 gates. MSI (medium scale integration) is used for chips containing 20-100 gates. LSI (large scale integration) ships contain 200-1000 gates, with the upper limit continually extending as VLSI (very large scale integration) became a reality. In the mid-1980s, ASIC (application-specific integrated circuits) ranged from 1000 gates to 20,000 gates (bipolar technology) or 200,000 (CMOS technology).

The AmZ8000 (Zilog) CPU contained 17.5K gates, the M68000 (Motorola) claimed to have 68,000 transistors. This, of course, pales against the 1999 chip sizes of 750,000 to 8 million gates (DSM (deep-submicron) CMOS technology), but it is where it all began.


For information about this file or to report problems in its use email

Copyright © September 1996, 1999, 2001, 2002 Donnamaie E. White White Enterprises