3 ms·

PDP11's were pretty great machines. Using RSTS/E (resource sharing time sharing... Eeeee!) you could really stretch things out. The process memory was limited

by rbobby 5y ago

PDP11's were pretty great machines. Using RSTS/E (resource sharing time sharing... Eeeee!) you could really stretch things out. The process memory was limited to 32kb (maybe 64kb... can't recall) and the OS's executable system supported seamless overlays. Basically as needed the OS would swap in different code segments (min 4k?). With careful planning you could build some pretty big applications... so long as you could get descent separation. You *could* swap overlays over and over and over... but this would kill any sort of performance... best to swap in a "AR" overlay, and when the user switches to "GL" swap that overlay in. The machine language was also pretty nice. Several addressing modes were program counter relative which was amazing (eg. JMP to 1500 words from current PC) . When I first learned that that windows loads an executable and then goes through and fixes up memory addresses so JMP instructions (et al) would work I could hardly believe it. If only the x86's had decent support for a PC relative addressing mode :) And the Digital Equipment Corp documentation! Glorious. A wall of shelves, 10 feet long, 7 feet tall, filled with 3 inch binders explaining everything in detail! Shivers!! Microsoft's online documentation is the closest thing in quality to the old DEC stuff.

monocasa 5y ago
Jumps have always been PC relative on x86. It's calls that can optionally not be. It's mainly the indirect calls that are troublesome. DEC documentation was something else though, agreed. My KD11-A came with everything you'd expect to be documented these, plus full schematics, plus microcode listing in source and flow chart form.
zh3 5y ago
PDP-11's (LSI-11's) were originally limited to 32k 16-bit words (64kb) but were exanded to 18bit addressing allowing up to 256Kb. The PC is only 16 bit though so 64k of code directly addressable at any one time (ignoring the reserved area for memory mapped hardware). The MMU was pretty simple so easy enough to access the extra memory without relying on the OS (simpler under RT-11 than RSTS/E). I still have shelves of DEC documentation (and a few PDP-11's).

by rbobby 5y ago

monocasa 5y ago

Jumps have always been PC relative on x86. It's calls that can optionally not be. It's mainly the indirect calls that are troublesome. DEC documentation was something else though, agreed. My KD11-A came with everything you'd expect to be documented these, plus full schematics, plus microcode listing in source and flow chart form.

zh3 5y ago

PDP-11's (LSI-11's) were originally limited to 32k 16-bit words (64kb) but were exanded to 18bit addressing allowing up to 256Kb. The PC is only 16 bit though so 64k of code directly addressable at any one time (ignoring the reserved area for memory mapped hardware). The MMU was pretty simple so easy enough to access the extra memory without relying on the OS (simpler under RT-11 than RSTS/E). I still have shelves of DEC documentation (and a few PDP-11's).