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 The 7502 was a 16-bit computer from 1974 
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Joined: Wed Jan 09, 2013 6:54 pm
Posts: 1419
I thought the numeric relationship to the 8 bit 6502 quite amusing!

ICL introduced the 7500 series in 1974, probably, and there was a series of machines which seem to be small computers, smart terminals, or workstations, depending on how you look at it. See below.

An interesting register file, dual-ported by replication, and using the two copies to assist in interrupt handling:
To improve normal processing speed, 7502 used two sets of 16 bit registers. Each set had 16 registers, with 15 being used for general purpose operations, and the last one used as an instruction pointer. All writes went to both register sets, but reads were taken from one set with a simultaneous read of a different register being possible from the second set. When an interrupt occurred, one register set was frozen, and the second set was diverted to process the interrupt. After the interrupt was completed, the frozen set was copied back to the second set to resume normal operation. During interrupt processing, special functions allowed the frozen registers to be backed up to memory and different contents to be loaded to switch to a different process. The block manipulating functions described above were able to accept an interrupt part way through their execution so that manipulating very large blocks did not adversely affect interrupt processing.

Here's the physical design - all text from wikipedia.

7502 consisted of a system enclosure containing up to eight PCB's (CPU card, memory cards, peripheral controllers and video cards). It was similar in size to a desk side or tower PC, but was mounted horizontally. As it was intended to function in an office environment, steel-framed, wood-veneered cabinets and furniture were available for the processor and peripheral units. The 7502 system enclosure had two levels to include space for the dual, 8-inch floppy disc unit. The interior of the cabinet was covered with acoustic-absorbent foam material to cut the noise from the cooling fans. The maximum connectivity was 8 x 7561 VDU stations and four serial printers, but in the early systems it was necessary to reduce the VDU attachments if floppy disc storage was attached. The rear of the 7502 system carried the connectors for VDUs, modem and serial printers and a set of 8 "engineer's switches" which could be used to input data and set options for "teleloading" software.

7501 and 7502 were functionally the same and shared identical interfaces and system software. A 4 Kbyte Read-Only Memory (ROM) in the normal address space provided a system bootstrap capable of downloading the operating software over the normal synchronous communications line, loading or dumping from/to local floppy disc or providing a local engineer's console. For diagnostic purposes an Engineer's Test Unit could be installed between the CPU card and the backplane. This gave the engineer full capability of reading and writing registers or memory and single-stepping machine code or CPU microcode. A digital cassette tape device could be used to load test or operating software.

Thu May 21, 2020 5:40 pm
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