Most of those who have heard of the 1900 series, and certainly those who have ever used them, will have head of the term 'executive'. Known as 'exec' for brevity it was an intrinsic and essential part of any 1900 system.
What exactly is it?
A simple question, but the answer is not equally simple: -
On some systems 'exec' was the entire Operating System. On others, those running GEORGE 3 or GEORGE 4 it was a lot less, in these cases it was more akin to a BIOS (Basic Input Output System) or a set of device drivers along with a process scheduler.
For different models of 1900 hardware, a different exec was required. This was well before the days of plug and play and memory (core) was expensive. This meant that users wanted exec to be both powerful and small, as it was always resident in their expensive memory. For instance the smallest of the 1901A based systems, which was operated by setting switches, had 4KW or (rarely) 8KW of store and exec would take up maybe 1.8KW to 2KW. On the high end machines like the 1904A a manual (i.e. non-GEORGE 3 or 4) system the exec could occupy 8 or more KW.
Store was so precious that exec was compiled specifically for each configuration or site. Only the 'device drivers' for peripherals that existed on the site or software options that were essential were included in the exec build. If you obtained a new printer, then you needed an updated exec. Plugging a card reader into a different I/O channel (a large fat cable and a 75-way plug) was not going to work unless the exec had been compiled with the correct model of card reader on that channel.
There have been a large and potentially bewildering list of different executives, more in the early days where to save store a machine capable of running 4 programs at a time would run maybe a single-programming or dual programming exec in order to reduce the overhead of exec and to provide more space for the user programs themselves.
What Execs do we have?
| Exec Name | Machine | Store Size (KW) | |
| E1HS | 1901A | 4 | Manual single-program exec, using paper tape or cards only. |
| E1DS | 1901A | 8, 12 or 16 | Single programming exec, with 4 x 7-track MT and 4 x EDS8 disc, along with Line Printer and Card Reader |
| E4BM | 1904/5/9 | 16, 24 or 32 | Multiprogramming exec, 4 programs each with up to three members (threads). 8 x 1974 7-track MTs, mixture of printers, punched card and paper tape devices, along with basic comms. |
| E6RM | 1904E - 1904S | 32 - 256 | Multiprogramming exec, unlimited (bar running out of store, peripherals etc.) programs each with up to 5 members. |
| EWG3 | 1904E - 1906S | 64 - 256 | Exec for GEORGE 3, this exec is really only an I/O monitor or BIOS, GEORGE 3 managed everything itself. |
| EWG4 | 1904A - 1906S | 64 - 256 | Exec for GEORGE 4. GEORGE 4 was GEORGE 3, but with the necessary support for the paged hardware environment. |
| E3NG | 1902A - 1903S | ? | Exec for GEORGE 3 to run on the Stevenage Machines (1902A, 1903A, 1902S, 1903S, 1902T) ??? |
What Execs don't we have?
| Exec Name | Machine | Store Size (KW) | |
| E3RM | 1902A - 1903S | <= 128 | A manual exec similar to E6RM for the smaller Stevenage machines |
| E6BM | 1904E - 1904S | 32 - 256 | A 16-programs, similar to E4BM |
| Many others | |||
Ever increasing details can be found by following the links on the left.
Last updated: 26-Apr-2025