Friday, 8 May 2009

16Kb memory expansion for ZX81

The original sinclair ZX81 has only a 1kb memory of RAM, so it was quite difficult , but not impossible , to produce any big program, obviously soon there was memory expansion for them and one of those modules was the 16Kb memory extension (usually any image of this blog , compressed and so on it's too big to fit in those memory modules, but in a few years ago 16 kytes of RAM where a lot of space .

As a curiosity , the three first models of sinclair used the same processor, the zilog z80, so where able to access up to 64Kb of RAM (2^16) , so with the different size in their ROM size (BASIC interpreter, Input/Output routines, etc), produces that the total RAM size of the machines where bigger in the first models (up to 60Kb in the sinclair ZX80, 56 kb in the sinclair ZX81 and only 48Kb in the last model sinclair Spectrum.

After the sinclair spectrum the other models (sinclair spectrum 128, +2, and +3 ) used a technique named "pagination" to share a memory space between different memory banks allowing to have more than 64kb, but not at the same time.

casio FX-702P

Another calculator from casio, with a BASIC interpreter allowing to develop your own programs, this model has the unusual feature of not using a QWERTY keyboard, and keys are aligned following their alphabetic order in a 7x4 rows

Sinclair ZX-interface 2

Sinclair made some interfaces to enhance the posibilites of the most successful product, the Sinclair Spectrum , those interfaces include the interface 1 (allow to connect to microdrive units and serial RS-232 interface ) , the ZX-printer interface (thermal printer and this interface, named "interface 2".

This interface allow to connect two joystick to the spectrum , using the "Sinclair type joystick", that usually links the numeric keys to the joystick and the other facility provided by this interface was the use of memory cards with games and utilities that allow to start running applications and programs without loading them from tape.

Unfortunately the cartridges were quite expensive so few games were produced in this format, also it was ROM (read only memory) so people can't save their own games in them.