[cctalk] Re: Bendix G-15 Restoration

*From:* Jon Elson via cctalk <cctalk(a)classiccmp.org> *Subject:* [cctalk] Re: Bendix G-15 Restoration *Date:* Monday, October 10, 2022 at 7:35 AM *To:* Christian Corti via cctalk <cctalk(a)classiccmp.org> *Cc:* Jon Elson <elson(a)pico-systems.com> Not quite. A G-15 instruction had two fields, T and N, each seven bits, that looked like sector addresses, and you could often think of them as sector addresses, but doing so carelessly could get you into trouble. N is the easiest one to understand. That was the sector "address" of the next instruction to be executed, which except in the case of two instructions had to be in the same line (track) of the drum as the current instruction. The problem is that the field could hold values from 0-127, but the words around the drum were numbered 0-107, and it was perfectly valid (and sometimes even useful) to specify a value of N in the range 108-127. What actually happened is that the processor subtracted the N value from the word number of the current instruction (termed L), stored the truncated difference in a seven-bit field of a hidden register on the drum named the Command (CM) register, and then as each word passed the read head of the drum, counted that field up until it overflowed its seven-bit size. At that point the drum was in the correct position to read the next instruction. Actually, it was a bit more complicated than that, because the arithmetic in CM was being done modulo 128, but the word addressing was modulo 108. The processor handled that by adding an additional 20 to the CM field if the drum passed word 107 before the count overflowed. There was an additional complication in that the adjustment did not take place if you executed an instruction from word 107, in which case the programmer had to bias their timing numbers by 20 instead to accomplish the adjustment. It was a lovely machine to program. You can read how the sausage was made in the Theory of Operation manual, section C-17, starting on page 40 (page 46 of the PDF): http://bitsavers.org/pdf/bendix/g-15/60121600_G15_Theory_Of_Operation_Nov64… There were 20 108-word lines (0-19) and four 4-word lines (20-23) on the drum that software could access, but instructions could be executed only from lines 0-5, 19, and 23. A register in the processor determined which line was currently selected to execute instructions, so execution stayed on that line until it was changed by either a Select Command Line/Mark Exit or Select Command Line/Return Exit instruction. Those were the only ways to branch from line to line. The T field in an instruction can be thought of as the operand address, and often effectively was, but you had to be careful with that idea, too. It was called T because it was the "timing number" for the instruction's execution. Instructions had two modes, Immediate and Deferred, determined by the I/D bit in the instruction word. Deferred mode is easier to understand. Like N, the difference between it and L was stored in a field of CM and counted up until it overflowed. At that point the processor would access the word(s) under the read and/or write head on the line(s) selected by the S and D fields of the instruction. In this mode, T effectively told the processor how long to wait until the operand word(s) were in position, so it was very much like an operand address. In Immediate mode for most instructions, however, the operand data started at L+1, the word number immediately after that of the instruction. T specified how many words the instruction would process, but in a weird way -- processing started with the word at L+1 but ended at T-1. A really lovely machine to program. In Immediate mode for a few instructions (multiply, divide, the shifts), T actually specified the number of word-times the instruction would take, but instead of processing words from the storage lines on the drum, it processed them in the double-precision registers, which were also on the drum. For shifts, each two counts of T would shift one bit; for multiply and divide, each two counts of T would develop an additional bit of the result. See the ToO manual, section D-10, D-11, D-12, and D-13 starting on page 53. So that's why I say the G-15 didn't have addresses. But then, it didn't really have op codes, either. For some insight into how advantageous use could be made of non-obvious values for the N and T fields in G-15 instructions, see this blog post describing a program that would clear the G-15 drum memory, its registers, and a couple of other things in just four words: https://retro-emulation.blogspot.com/2022/06/ Paul