// format for hand-rolled floating point used in PDP-7 version
struct flt {
char exp; // exponent
int m1; // signed 36-bit mantissa (0-18)
int m2; // 18-36
};> Also during 1969, Thompson developed the game of ‘Space Travel.’ First written on Multics, then transliterated into Fortran for GECOS (the operating system for the GE, later Honeywell, 635), it was nothing less than a simulation of the movement of the major bodies of the Solar System, with the player guiding a ship here and there, observing the scenery, and attempting to land on the various planets and moons. The GECOS version was unsatisfactory in two important respects: first, the display of the state of the game was jerky and hard to control because one had to type commands at it, and second, a game cost about $75 for CPU time on the big computer. It did not take long, therefore, for Thompson to find a little-used PDP-7 computer with an excellent display processor; the whole system was used as a Graphic-II terminal. He and I rewrote Space Travel to run on this machine. The undertaking was more ambitious than it might seem; because we disdained all existing software, we had to write a floating-point arithmetic package, the pointwise specification of the graphic characters for the display, and a debugging subsystem that continuously displayed the contents of typed-in locations in a corner of the screen. All this was written in assembly language for a cross-assembler that ran under GECOS and produced paper tapes to be carried to the PDP-7.
Add to that doing bit-banging on pins to simulate serial protocols with timing loops; storing things in odd places like the file buffer on a time-share system (used to run 20+ terminal sessions on a single 1MHz processor!) or the previous line on a programmable terminal (up-arrow and transmit-line to get it back).