ENIAC ....

On 29 Jan 2011 at 14:22, Tony Duell wrote: Why any (visible) registers at all? Memory-to-memory was the basis for several IBM computers. The aforementioned 1620 was a 2-address machine with variable word length with no visible registers (the P- counter was not readable by a program). Word length was essentially limited by the size of a bank of memory (about 20K decimal digits). Word length was at least 2 digits, with an extra bit serving the dual purpose of a sign (if set on the low-order digit of a word) or a demarcator (if set on the high-order digit). While it's true that the 1620 used 5 digit address registers for memory access and the P-counter, none was readable by a program. It sounds like it was clumsy, but the architecture was very friendly to hordes of early 1960s students. Being decimal, it was a system whose learning curve was not steep. The usual unit record interfaces, in addition to a console typewriter and an optional disk interface (to the 1311) were present. Arithmetic was performed by table lookup, with the addition and multiplication tables located in low memory and alterable by user programs. Dijkstra hated the machine, but it was popular and as the 1710, was one of IBM's first industrial process control computers. --Chuck