27 bits, one's complement. The opcode layout is 6 bits for operation, 2 for addressing mode, 2 for controlling conditional execution, 2 for specifying whether/how to set the condition flag, and 15 bits for address or immediate operand. There's a short description in Wikipedia (both the EL-X1 and the EL-X8). The two share the same basic instruction set, the X8 adds float and has a different I/O system with a coprocessor (CHARON). The X1 has what may be history's strangest addressing mode ("C" mode). One handy thing you could do with this is have instructions that are both conditionally executed and condition-setting, which lets you do Boolean operations without explicit AND or OR instruction use. For example, if you wanted to know if X is zero and Y is >= 0, you could write: A=x, Z Y, A=y, P and end up with the condition flag set to "yes" if that composite condition is true. (If you needed OR rather than AND, the Y would simply become N in the second line.) For a pretty detailed description of the X1, see Dijkstra's Ph.D. thesis, which is online in the U. Texas EWD archive. The X1, incidentally, was as far as I can determine the first commercial computer with interrupts standard. (TX-0 did interrupts slightly earlier and IBM offered interrupts as an option at about the same time as the X1, I believe.) Also, X1 had what you might call a BIOS, in ROM. Core ROM, that is -- different from "core rope" and somewhat more efficient. paul