During the life of the 1900 Series machines there were a number of approaches to the provision of floating point instructions: -
The first set of choices was simple: -
It appears that every 1900 CPU was available with some FP facility, it may be doubted that some of the smallest 1900s CPUs were actually sold with the FPU as there could well have been more logic involved in the FPU than in the CPU itself.
1900 CPUs and the available options
The First series
The first 1900 introduced was the 1904, and there was a standard for those CPUs produced in West Gorton (WG) for the fitting of an FPU to be indicated by adding 1 to the model, therefore a 1904 with an FPU was called a 1905. - This convention did not apply to the CPUs manufactured in Stevenage (SV).
| Model | Where | FPU Option Description |
| 1904 (1905) | WG | A hardware FPU the provided six of the eight FP orders in hardware. The FLOAT and FIX orders were implemented as extracodes in exec. |
| 1902, 1903 | SV | An FPU was available, and was a common option for the 1901, 1902 and 1903. |
| 1901 | SV | The smallest 1900, max 16KW had an FPU offered as an extra on the Stevenage MDU (Multiply and Divide Unit), the CPU itself leaving integer multiplication and division to extracodes. |
| 1906 (1907) | WG | The biggest of the first series machines, based on the logic from the Orion. The 1907 FPU implemented seven of the eight FP orders, leaving FIX to executive. |
| 1909 | WG | A 1905 fitted with a slow 6uS store to reduce the price, possibly to attract academic usage. |
The Second Series (E/F)
This series were all from West Gorton. Using much the same logic as the first series, they differed with the addition of extended data mode (EDM or 22AM) which allowed addressing up to 128KW (256KW???) and EBM (Extended Branch Mode). They were all 1904 or 1905 depending on the FPU's presence, but had a suffix of E or F. The significance of the E/F designation was as follows: -
| Model | Where | FPU Option | Description |
| 1904E | WG | none/extracode | 1900 CPU with EDM&EBM, 6uS store |
| 1904F | WG | none/extracode | 1900 CPU with EDM&EBM, 2uS store (F=Fast???) |
| 1905E | WG | FPU from the 1905 | 1900 CPU with EDM&EBM, 6uS store |
| 1905F | WG | FPU from the 1907 | 1900 CPU with EDM&EBM, 2uS store (F=Fast???) |
| 1906E | WG | none/extracode | Two 1904E machines as a multiprocessor |
| 1906F | WG | none/extracode | Two 1904F machines as a multiprocessor |
| 1907E | WG | none/extracode | Two 1905E machines as a multiprocessor |
| 1907F | WG | none/extracode | Two 1905F machines as a multiprocessor |
The Third series
The final 1900 Series was in many was similar to the second, except that they were implemented using TTL (Transistor-Transistor-Logic) chips and not from logic built from discrete components - DTL (Diode-Transistor-Logic) this allowed a speed increase not available from the older logic without massive re-design and an increased power consumption. It is to be noted that the convention where WG machines indicated the presence of an FPU with an odd numbered model was dropped. THis series was introduced around 1968 with the 1904A, then the 1904S and 1903T. The 1904S was a faster 1904A, with selected parts of the CPU employing the faster Schottky TTL chips and a MOS store. At the same time Stevenage produced their A-series to replace the 1901, 1902 and 1903 machines with the 1901A, 1902A and 1903A.
There was also the 1906A and 1906S machines. Right at the top of the 1900 range these were mighty beasts that used ECL (Emitter-Coupled Logic) to provide speed unobtainable from TTL at the time. The had an Extended Floating Point system, giving considerably more precision (4-words long - 96 bits). Whilst normal FP programs would run happily and fast on these machines, they were a natural for academia and other specialist usages.
| Model | Where | FPU Option Description |
| 1901A | SV | A hardware FP was available - apart from
being implemented in TTL, it provided FP instructions which ran within
the CPU and used its 24-bit hardware registers, along with some extra
registers in the so-called CCF/SCF options. CCF = Commercial Computing Facility (added Multiply & Divide orders inter alia) SCF = Scientific Computing Facility (added FP, required the CCF hardware to be in place) |
| 1902A | SV | |
| 1903A | SV | |
| 2903 | SV | OK, that '2' at the start is not a typo, and it was not a 2900 machine either. How it got its name is another tale (See Resurrection mag???). However the 2903 was a bit of a 'Skunk Works' project that used the MICOS 2 microcode engine used as part of the 2900 series I/O <stuff>, but with microcode (soft-loaded) that allowed it to emulate a low-end 1900 Stevenage style system. On this machine FP was implemented (I think) either as extracodes in the 1900 realm or by some extra firmware in the MICOS 2 store. THis was referred to as the FAST FP option. |
| 1904A | WG | Optional, This TTL FPU implemented all eight of the FP orders. |
| 1904S | WG | Optional, This TTL FPU implemented all eight of the FP orders. There appears to be some differences in the logics for the 4S FPU, this could simply be engineering fixes being built-in or possibly some the this FPU was enhanced to use Schottky TTL. |
| 1903T | WG | Despite being called a 1903T, which would imply a Stevenage product, this was a 1904A machine fitted with a cheaper (slower) store and sold as a placeholder at the top of the Stevenage range to retain customers. |
| 1906A | WG | Almost identical apart from the introduction of the Plessey 'Plated wire' 240nS store vs the conventional 650nS core store on the 1906A. |
| 1906S | WG | The FPU was implemented in ECL and allowed greatly increased precision and speed over their TTL predecessors. |
You can select from the links on the left to drill deeper into the depths of 1900 FPUs.
Last updated: 25-Apr-2025