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Fig. Crtm-4. Control part showing Crtm-1 fetch, decode, and execute processes for instruction interpretation.

RTM IMPLEMENTATION OF Crtm-1

Figure Crtm-5 shows the RTM diagram for the small general purpose, stored program computer that we have been calling Crtm-1. It was initially constructed as an application experiment to demonstrate the feasibility of RTM's and to investigate systems problems. The process of specifying the machine took approximately two hours (with three people). The computer was wired, and aside from minor system/circuit problems (for which the experiment was designed) the computer operated essentially when power was supplied, since there were no logic errors.

The computer was designed for an actual application which had 300 constants, 600 control steps and 16 variables. (An alternative approach would have been to hard-wire the 600 control steps directly, thereby operating faster, but being more expensive and less flexible.) The computer has only 24 K(evoke) and 10 K(branch) modules. (By way of comparison, RTM interpreters require about 90 control modules, 2 DMgpa's, and core memory to emulate the PDP-8 minicomputer.) Since the price ratio of a single hardwired control to a single read-only memory control word is approximately 10:1, and since the overhead price of a 1000-word read-only memory is about the same as 100 controls, it was cheaper in the above application to use RTM's to first build an interpreter, (i.e., a stored program digital computer) and then let the computer program execute the control steps.

The data part of the machine is shown in the upper right of Figure Crtm-5. Three DM-type RTM modules hold the processor state and temporary registers. The processor state, that part of memory accessible to and controlled by the program, includes: A, the accumulator; P, the program counter; and L, the register used to hold subroutine return addresses (links). The temporary registers

300

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