PDP-8/I

PDP-8/I
PDP-8/I front panel
Year Introduced:	1968
Form Factor:	minicomputer
Word Size:	12
Logic Type:	TTL
Design Type:	clocked random logic
Clock Speed:	333KHz
Memory Speed:	1.5 μseconds
Physical Address Size:	32KW (requires optional MC8/I)
Virtual Address Size:	4KW
Memory Management:	bank selection, CPU mode
Bus Architecture:	negative I/O bus
Operating System:	TSS/8. Disk Monitor System
Predecessor(s):	PDP-8
Successor(s):	PDP-8/E

The PDP-8/I was introduced in 1968 as the successor to the PDP-8. It was constructed out of TTL ICs on M-class FLIP CHIPs - mostly early standard-length single-height (width) ones, with a few dual-height. It was the first DEC computer to be built using ICs.

Options included:

• KA8/IB Positive I/O Bus Interface, to allow use of newer PDP-8 devices
  • DW08A Negative I/O Bus Conversion Option (on systems with a KA8/I, allows use of older PDP-8 devices)
  • DW08B Positive I/O Bus Conversion Option (on systems without a KA8/I, allows use of newer PDP-8 devices)
  • DW8-E OMNIBUS Interface (converts the positive bus of an 8/I)
• MC8/I Memory Extension Control, bank switching needed to support more than 4K words of memory
• MP8/I Memory Parity
• KE8/I Extended Arithmetic Element, which supported hardware integer multiplication and division, one-bit double-word shifts, and normalization
• KT8/I Time Sharing Hardware Modification, which allowed the computer to operate in either Executive Mode or User Mode

It could perform an addition to the accumulator in 3.0 μseconds, and a 12 by 12 bit multiplication with 24 bit result in 6.0 μseconds, using the math extension hardware.

The MC-8/I had three 3-bit registers: the Instruction Field and associated Instruction Buffer, and Data Field, which could be loaded either manually from the front panel, or under program control (by the Change Instruction Field - something of a misnomer, as it was actually the IB which was loaded - and Change Data Field instructions). The IB contents were loaded into the IF by a jump or subroutine call.

Images

PDP-8/I of the Göttingen Computer Club in Göttingen, Germany

Another image

External links

• PDP-8/I - Original PDP-8/I documents