IMLAC PDS-1
An Imlac PDS-1 was a very early workstation (1970, well before the Xerox Alto), with a vector graphics display. It generally did not have any secondary storage, and was only a terminal on a larger system.
This computer was host to the first version of the game Maze War. As a terminal, it was used to develop the game Zork, and edit the manuscript for the book Gödel, Escher, Bach.
Differences between Alpha and Graphics machines
The first type of the Imlac PDS-1 was called an Alpha machine, and had a somewhat less capable display processor than the later Graphics machines. There is no surviving documentation for the Alpha machine, but the details have been reverse engineered by Zachary Harper from the listing for the Freeway Crossing game.
On an Alpha machine, the D LDXA and D LDYA instructions load bits 4 to 10 into the accumulator MSB. On a Graphics machine, bits 6-11 load into the MSB. On both machines, bits 12-16 are loaded into the LSB, and the least significant "½ point" bit is cleared.
On an Alpha machine, beam deflection from the MSB is scaled up by 9 LSB units. On a Graphics machine, the MSB is scaled up by 16 LSB units. (The LSB least significant bit is 0.5 units.)
On an Alpha machine, incrementing or decrementing the LSB does not add any carry to the MSB. The LSB can go outside 0-15 units; exactly by how much is unknown. On an Graphics machine, going outside 0-15 wraps around and carries to the MSB.
On an Alpha machine, incrementing or decrementing the MSB is done according to the scale factor. On a Graphics machine, it's done by 1.
