PDP-4

PDP-4
Manufacturer:	DEC
Year Introduced:	1962
Form Factor:	small computer
Word Size:	18 bits
Logic Type:	germanium transistor System Modules
Memory Speed:	2 μsec (access time) 8 μsec (read/write cycle time)
Physical Address Size:	15 bits (32K words - requires Type 16 Memory Extension Control)
Virtual Address Size:	13 bits
Predecessor(s):	PDP-1
Successor(s):	PDP-7
Price:	US$65K

The PDP-4 was a small computer produced by Digital Equipment Corporation; DEC's second production computer, and another 18-bit machine, like its predecessor, the PDP-1. It too had a load-store architecture, with a single accumulator, and a directly-accessible link (carry) bit. Introduced in 1962, it was originally intended principally for process control applications; approximately 54 were sold.

The instruction set, although similar to that of the PDP-1, was simplified; removing less-used complexity allowed it to be physically smaller (less than half the size) and cheaper (roughly half the cost) than its predecessor. The resultant machine had 5/8 the performance of the PDP-1, but the differential was not large enough to boost sales to the hoped-for level. PDP-1 main memory and peripherals could be used on the PDP-4, though.

The PDP-4A included 1KW of core main memory; the PDP-4B had 4KW, but was otherwise identical. A Type 17 (later Type 134) Core Memory Module added another 4KW to a PDP-4B; after that, a Type 16 Core Memory Extension Control allowed (via bank switching) use of up to three additional 8KW Type 135 Core Memory Modules.

The optional Type 18 (later Type 22) Extended Arithmetic Element added 23 instructions to facilitate high speed multiplication, division, normalization and shifting. "The EAE contains an 18-bit register, the Multiplier Quotient (MQ); a 6-bit register, the Step Counter (SC); and a 3-bit Instruction Register."

Standard peripherals included a paper tape reader. Optional peripherals included a terminal (a Teletype Model 28), several different graphics displays, a light pen, line printer, paper tape punch, punched card reader and punch, and magnetic tape controller and drives. The Type 24 Magnetic Drum System could hold either 16KW, 32KW or 64KW of secondary storage.

Technical details

The instruction format consisted of a 4-bit basic operation code, a 1-bit indirect address bit, and a 13-bit memory address. The augmented instructions all share an operation code. In them, the address field is not used for an address; instead, each bit of the field can specify a separate operation, and generally more than one bit can be set at a time (except for most of the skip operations). The Program Counter is also 13 bits long.

The PDP-4 supported both one's complement and two's complement number representations; the latter was better for multiple precision calculations. When used with indirect addressing, memory locations 010-017 were auto-incrementing; this allowed leaving out index register functionality.

Direct Memory Access for high-speed devices was provided by the three cycle data break mechanism. An 'event counting' mechanism was also provided, which could keep a count of an external events without using an interrupt; it read out a fixed main memory location, incremented it, and wrote the updated value back.

It was constructed out of 4000-Series System Modules, which used germanium transistors.

Instruction set

The PDP-4 instruction set, which is largely the same across the family. The PDP-15 added an index register.

Octal	Binary	Mnemonic	Meaning
00	0000	CAL	Call subroutine
04	0001	DAC	Deposit accumulator
10	0010	JMS	Jump to subroutine
14	0011	DZM	Deposit zero to memory
20	0100	LAC	Load accumulator
24	0101	XOR	Exclusive or
30	0110	ADD	One's complement addition
34	0111	TAD	Two's complement addition
40	1000	XCT	Execute
44	1001	ISZ	Increment and skip if zero
50	1010	AND	Logical and
54	1011	SAD	Skip if accumulator different
60	1100	JMP	Jump
64	1101	EAE	Extended arithmetic element
70	1110	IOT	I/O transfer
74	1111	OPR	Operate

Further reading

• "PDP-4 Manual", F-45, Digital Equipment Corporation, 1962

External links

• PDP-4 - Original PDP-4 documents
• PDP-4 Manual, 1962
• Programmed Data Processor-4 - marketing brochure from DEC
• PDP-7 - A few PDP-4 paper tape images in the PDP-7 directory
• C. Gordon Bell, Gerald Butler, Robert Gray, John E. Mcnamara, Donald Vonada, and Ronald Wilson, The PDP-1 and Other 18-Bit Computers, in C. Gordon Bell, J. Craig Mudge, John. E. McNamara, Computer Engineering: A DEC View of Hardware Systems Design, Digital Press, Bedford, 1978