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The PDP-1 used an 18-bit word size and had 4096 words as standard main memory (equivalent to 9,216 eight-bit bytes, though the system actually used six-bit bytes), upgradable to 65536 words. The magnetic core memory's cycle time was 5 microseconds (corresponding roughly to a clock speed of 200 kilohertz); consequently most arithmetic instructions took 10 microseconds (100,000 operations per second) because they used two memory cycles: one for the instruction, one for the operand data fetch. Signed numbers were represented in one's complement. The PDP-1 had computing power roughly equivalent to a 1996 pocket organizer and a little less memory.
The PDP-1 used 2,700 transistors and 3,000 diodes. It was built mostly of DEC 1000-series System Building Blocks, using micro-alloy and micro-alloy diffused transistors with a rated switching speed of 5 MHz. The System Building Blocks were packaged into several 19-inch racks. The racks were themselves packaged into a single large mainframe case, with a hexagonal control panel containing switches and lights mounted to lie at table-top height at one end of the mainframe. Above the control panel was the system's standard input/output solution, a punch tape reader and writer.