Hardware - Explaining the BitSlice Design

A project log for Suite-16

Suite-16 is a 16-bit cpu built entirely from TTL. It is a personal exploration of how hardware and software interact.

monsonitemonsonite 10/15/2019 at 22:140 Comments

In my last project log I discussed the instruction set for Suite-16, and actually found time to implement some of it in a C code simulator. This is an ongoing work in progress - and will be discussed in a future project log..

In this log, I'm going to attempt to clarify the hardware design and explain what is meant by a bitslice.

Bitslice design is a means of partitioning complex logic circuits into manageable modules. It has it's origins in the early 1960s and was used extensively in computers such as the PDP-8 (1965) and the Data General Nova (1969).

It's based on the premise that when you perform bitwise logic on multi-bit operands (say 8-bit or 16-bit) that each bit can be treated as a separate operation - with no dependencies to bits either side to the one that is being processed. 

In short, if you wish to perform a 16-bit  bitwise AND on two operands A and B, then you could do this with four identical circuits - each of which could AND four bits. By stacking four of these identical 4-bit AND modules next to each other, you can perform a 16-bit AND.

When it comes to arithmetic operations such as ADD and SUB - conveniently the manufacturers of TTL chips designed a 4-bit adder  (usually the 74xx283).  As a single IC this can add two, 4-bit numbers, with internal connections for the Carry signals from one bit to the next. If you take four of these chips and connect the CarryOut (CO) of the lesser stages to the CarryIn (CI) of the next stage - you can build up larger adder circuits for 8, 12 and 16-bit designs.

TTL first appeared in the early 1960s - demand driven by the aerospace and missile industry. Each IC was restricted to at most a few hundred transistors - medium scale integration or MSI. The designers of those early 74xx series logic devices tried to cram as much as they could into the much cheaper 14 pin and 16 pin packages.  Later on, the devices became much cheaper and were widely adopted by the computer industry. Now, some 50 years later since the commercial introduction of affordable TTL (in about 1969) that so many of the useful functions cater for a 4-bit wordsize and are still all available today - packaged in a 16 pin DIL package.

This is all great news for the designers of TTL computers - even in 2019.

Some of the 4-bit wide 7400 series ICs that will be used in Suite-16:

74xx157   Dual 4-input multiplexer

74xx157   Quad 2-input multiplexer

74xx161    4-bit presettable binary  counter

74xx173   Quad D-type flip flop

74xx193   4-bit synchronous binary up/down counter

74xx194   Universal 4-bit bidirectional shift register

74xx219   4-bit x 16 word RAM

74xx283   4-bit binary adder

74xx670   4 x 4 register file

A Bitslice  ALU based on Multiplexers

At the heart of any processor is the Arithmetic and Logic Unit or ALU.  As its name suggests, it performs arithmetical and logic operations on usually two binary operands. The ALU can be very simple - such as the PDP-8 which offered just ADD and AND of a memory operand with the Accumulator. It  also allowed the Accumulator to be inverted, and a carry bit added so that instructions such as subtraction could be synthesised. With inversion and bitwise AND, all of the other logic operations could also be created from small macros of instructions.

The ALU of the PDP-8 was designed at a time before integrated circuits were widely available and used a discrete diode-transistor logic called DTL.  Transistors were very expensive (about $20 in today's equivalent), and so were used very sparingly, with the diodes supplying much of the AND-OR logic. The PDP-8 was one of the first commercial bitslice designs, with 12 circuit boards stacked side by side in the rack - each of which contained 1 bits-worth of circuitry.

With the arrival of cheaper TTL, you could afford to design a more sophisticated ALU - and by the late 1960s you could buy a complete, multifunction 4-bit ALU - the 74181, packaged in a 24 pin package.  The 74181 was the computer design engineer's dream - and was widely used in minicomputers and controllers where multi-bit computation was needed.

Unfortunately the 74181 has been obsolete for many years, and is impractical to use for a new design. However there is a clever means to obtain almost all of the capability of the 74181 using a combination of 4-input multiplexers and an adder.

The proposed ALU will have the following instructions ADD, SUB, AND, OR, XOR with extensions for INV, INC, DEC and SHR (shift-right). 

I am deeply indebted to Dieter Muller of for the design of the ALU.  More than a decade ago, Dieter described a way of using low cost multiplexer ICs to create a very versatile ALU. 

Dieter has documented many years of TTL design from his own experience - and his pages on ALU design using multiplexers describes them as the "Tactical Nuke" of Logic Design.

I won't go into the full details here, partly because I will cover them in full in a later post - and also because Dieter more than adequately describes them on his own site (well worth the read). Needless to say, his design has been widely adopted as the coolest way to make a versatile ALU from simple TTL chips.

Suite-16 has been greatly inspired by Marcel van Kervinck's Gigatron, which uses an ALU based on Dieter Muller's design. Suite-16 embraces this design and packages it up into a form suitable for bitslice design.