|Manufacturer:||Digital Equipment Corporation|
|Year First Shipped:||February, 1970|
|Word Size:||18 bits|
|Logic Type:||TTL ICs|
|Instruction Speed:||1.6 μsec (basic)|
|Memory Speed:||0.8 μsec|
|Physical Address Size:||17 bits (128K words)|
|Virtual Address Size:||12 bits (direct), 15 bits (indirect), 17 bits (indexed)|
|Memory Management:||bounds register; base and bounds pair (both optional)|
|Operating System:||DECsys, Keyboard Monitor System, Foreground/Background System, DOS-15, XVM/DOS, XVM/RSX, XVM/MUMPS, Advanced Monitor System|
The PDP-15 was DEC's last 18-bit computer, and the only one implemented using integrated circuits. Its principal intended use was for real-time systems. A variety of models were offered, from the PDP-15/10 (with 4K words of main memory), to the PDP-15/40 (with 24K words, and two disks).
Both multiply/divide and floating point support were hardware options (the former being standard on all but the lowest model). The FP15 floating point unit was a complete separate processor, but shared the instruction set space with the basic CPU.
KM15 memory management included a boundary register to set the boundary between protected and un-protected memory, and two modes for the CPU. A memory relocation option, the KT15, with a base and bounds register pair, was also available.
Later models supported an interface (the UNICHANNEL-15, UC15) to a satellite PDP-11 (usually a PDP-11/05), through which other PDP-11-native peripherals could be supported, including DMA directly into the PDP-15's memory through the MX15-B Memory Multiplexer. These sometimes used the UNIBUS adaption for 18-bit mode, where the two parity lines were recycled into 2 extra data lines.
Instructions had a 4-bit opcode, one bit of indirect, one of indexing, and a 12-bit address field (which was used together with other information, such as the contents of the Index Register, to form the actual address of the operand):
Values for the Address Mode ('E') field are:
Of the 16 possible instructions, 13 used the format above; the other 3 used the non-opcode bits to 'microcode' (in DEC's then-terminology) multiple non-memory reference instructions into a single instruction word.
(All available online through BitSavers.)
- PDP-15 Systems User's Handbook: Volume I - Processor
- PDP-15 Systems User's Handbook: Volume II - Peripherals