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1980s Computers Explanation of terms

The FX-785P and FX-790P



Casio (Japan)


FX-785P and FX-790P

Date Launched



Approx £80 (FX-785P)
£100 (FX-790P)

Microprocessor type

HD61747B20 (Main CPU - responsible for BASIC control, the lower keyboard and digits 1 to 6 & 13 to 18 of the display)
HD61747B25 (Subsidiary CPU - responsible for calculation, I/O operations, the upper keyboard and digits 7 to 12 & 19 to 24 of the display)
Used CMOS technology allowing a long battery life of 100+ hours use from two lithium button cells.

ROM size

Not known

Standard RAM

2 kilobytes (FX-785P)
8 kilobytes (FX-790P)
The two models were otherwise identical.

Maximum RAM

10 kilobytes (FX-785P)
16 kilobytes (FX-790P)

Keyboard type

Moving rubber keys on the lower half (numeric and calculator functions).
Touch sensitive plastic membrane on the upper half (QWERTY section).

Supplied language

10 separate programs could be stored at once (subject to memory limits) and the calculator functions could be used in programs.
Programs were retained when the computer was switched off.

Text resolution

LCD display, 1 line of 24 characters

Graphics resolution

No graphics

Colours available


Example Screenshot

Casio FX-790P display
(Simulated) display of the FX-790P. The single line display meant prompts and output had to be abbreviated.


Beeper with two notes

Cassette load speed

Not known
Required an external cassette interface, the FA-5, which cost £27 and plugged into an expansion slot on the side.

Dimensions (mm)
Weight (grams)

142 x 71 x 18 folded

Special features

A pocket computer of folding design, similar in size to a pocket calculator.

Included a built-in assembler trainer. Curiously this was not an assembler for the actual processor but for an emulated simplified processor, intended for learning the techniques of assembly language programming.

Had a 'Memo' function which was actually a simple database.

Good points

Contained all the functions of an advanced scientific calculator, grouped on the lower half of the keyboard, in addition to being programmable in BASIC.
The maximum of 16KB of RAM was quite large for a pocket computer of the mid 1980s.
A portable till-roll type printer was available for £70.

Bad points

The membrane keypad had no tactile feedback and the folding design meant you had to hold the top half tilted up with one hand while pressing the keys with the other hand.
The BASIC was quite slow in operation.

How successful?

A number of different models of pocket computers were produced by Casio and Sharp in the 1980s but they never seemed to gain a large following.
One of the biggest limitations of all the early pocket computers was that the small displays only allowed you to see part of one line of a BASIC program at a time, and this made it difficult to follow the flow of a program.
The FX-790P was also sold by Tandy as the PC-6.


The idea of a fully programmable computer which was small enough to slip into a jacket pocket was an interesting one, which does not seem to have an equivalent in 2006. Current PDAs are of course much more powerful but not intended to be programmed by the user and often do not have keyboards, while a laptop is too bulky to carry unless you are definitely going to need it, and anyway has a battery life of just a few hours. I can certainly testify to the long battery life of an FX-790P - mine has had only occasional use since 1991 but the batteries were still just about working at the start of 2005!

Casio FX-850A later model, the FX-850, was of non-folding design and cost £90. It had 8KB of RAM, expandable to 40KB, and benefited from a two line x 32 character display. Its other notable feature was a built-in library of 116 scientific, mathematical and statistical formulae.

In late 2007, Calculators-Online (UK) are still selling the Casio FX-795P (which is the FX-790P but with 16KB of RAM) brand new for £130.

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