Bit slicing, although not called that at the time, was also used in computers before
large-scale integrated circuits (LSI, the predecessor to today's
VLSI, or very-large-scale integration circuits). The first bit-sliced machine was
Whirlwind I, built in 1946–1951. Its floor plan had a row of "
relay racks" (or "racks" for short) for each group of closely-related and highly-interconnected circuitry, such as the A row with the CPU registers and arithmetic circuitry. Within a row, the circuitry a single each bit position within a 16-bit word was in a separate rack, such as racks A0–A15 in the A row. Within a rack, there were panels holding the circuitry for a given function. The A row racks had, from top to bottom, panels for the Instruction Register ("Program Register" and A Register, the Program Counter, the B and I/O Registers, the Accumulator (where the arithmetic was done), and the Check Register and Comparison Register. This allowed each rack A0-A15 to be identical and each corresponding panel in these racks to be identical. Subsequent first-generation machines built with the bit slice concept included the
Memory Test Computer built at MIT as part of the Whirlwind research in 1952–1953, and the
EDSAC 2, built at the
University of Cambridge Mathematical Laboratory in 1956–1958. In second generation (discrete transistor) machines, bit slicing was used to partition circuitry into a row of identical plug-in modules, with each module holding one bit of each of several registers. One example was the
PDP-6, a 36-bit machine with 18-bit memory addresses, in which 9 modules of type 6203 held the 9-bit shift count and floating point exponent registers, 36 modules of type 6205 held the several 36-bit arithmetic registers, and 18 modules of type 6206 held the several 18-bit memory-address related registers. Prior to the mid-1970s and late 1980s there was some debate over how much bus width was necessary in a given computer system to make it function. Silicon chip technology and parts were much more expensive than today. Using multiple simpler, and thus less expensive, ALUs was seen as a way to increase computing power in a cost-effective manner. While
32-bit microprocessors were being discussed at the time, few were in production. The
UNIVAC 1100 compatible series mainframes (one of the oldest series, originating in 1962) has a
36-bit architecture, and the 1100/60 introduced in 1979 used nine
Motorola MC10800 4-bit ALU chips to implement the needed word width while using modern integrated circuits. At the time 16-bit processors were common but expensive, and 8-bit processors, such as the
Z80, were widely used in the nascent home-computer market. Combining components to produce bit-slice products allowed engineers and students to create more powerful and complex computers at a more reasonable cost, using off-the-shelf components that could be custom-configured. The complexities of creating a new computer architecture were greatly reduced when the details of the ALU were already specified (and
debugged). The main advantage was that bit slicing made it economically possible in smaller processors to use
bipolar transistors, which, at that time, switched much faster than
NMOS or
CMOS transistors. This allowed much higher clock rates, where speed was needed for example, for
DSP functions or
matrix transformation or, as in the
Xerox Alto, the combination of flexibility and speed, before single-chip CPUs were able to deliver that. == Modern use ==