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Byte_1975-09

Byte Magazine
September 1975


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Byte Magazine
September 1975

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Byte Magazine
September 1975
page 13

inferior. In terms of numbers, the 8080 may require 10 to 15 percent more memory bits and the 8008 20 to 40 percent more. Note that these figures are based on optimized programs written by experienced programmers. The spread can be much greater with inexperienced programmers or hastily written programs. It is also interesting to note that instruction set organization and memory efficiency are usually conflicting requirements. This is because many of the lesser used possible operation combinations have been culled from a memory efficient set in order to reduce the number of bits required to encode the instruction. Implied operands are also utilized in order to free up bits for other uses. Experienced programmers are able to plan ahead and avoid having these restrictions become restrictive. The 8008 and 8080 are as good as they are because many of the instructions are a single word (8 bits) long whereas the minimum instruction length in the IMP-16 is 16 bits. This is somewhat offset by the three word (24 bit) instructions of the 8008 and 8080 which in most cases would only require 16 bits in the IMP-16. A fringe benefit of high memory efficiency is that the shorter programs will load faster regardless of the loading method.

Time efficiency is by far the best on the IMP-16 with the 8008 a distant second and the 8080 a slightly poorer third. On a classic minicomputer, a machine cycle was the same as a memory cycle in most cases. As a result, a time efficient instruction set meant a faster machine without faster hardware. Microcomputers on the other hand may have very few of their machine cycles being memory cycles. As a result, time efficiency may have little relationship to actual machine speed but does represent the potential speed with an optimized CPU. Time efficiency can be important in multiprocessor systems with a shared memory where more memory cycles increase the probability that a CPU will have to await its turn. The IMP-16 has a high time efficiency mainly because twice as much data is fetched in each memory cycle. Futher improvement is due to the instruciton set power, requiring fewer instructions to be fetched. The 8080 has poorer time efficiency that the 8008 mainly because the stack is in memory. A subroutine call, for example, requires 5 memory cycles, 3 to fetch the instructions and two to stack the return address.

Historically some minicomputers were better at handling character oriented tasks an others were well adapted to number crunching tasks. Microcomputers are no exception. Most micros have been optimized for character handling because of expected high usage in terminals and the 8008 and the 8080 belong to this class. The IMP-16 on the other hand is much better at numerically oriented tasks and was aimed more toward machine tool control and industrial monitoring. Interestingly, use of the extended instruction set on the IMP-16 improves both character handling and arithmetic capability.

How about Running System Software?

One performance area of interest to hobbyists is the suitability of a machine for running a BASIC system. The IMP-16 and the 8080 are about equal in their ability to compile BASIC quickly but the IMP-16 without the extended instruction set may
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