Not quite a PC XT, not quite a PCjr -- designed to be cheap like the latter, but has none of its extensions (no fancy sound or 16-color mode).
What's it like:
- Has single 16 MHz crystal, thus the CPU and timer clocks are higher (5.33 and 1.33 MHz). - No DMA. - No keyboard controller, key matrix is connected to second i8255. - Base system has 128K memory on board (96K available to the user); with the expansion installed, user memory is 608K, not 640K. - CGA implementation has more video memory (32K), which is shared with system memory; there is space for 4 fonts in the chargen ROM, but only one is present (at 0). - Has two cartridge slots for expansion. There are at least three (extra 512 KB of memory, a BASIC ROM and a FDC).
These are incompatible on the low level, but have compatible BIOS interfaces: - FDC -- uses a FD1793. - Serial port -- uses a i8251. - Parallel port. - Cassette interface.
Software: SF-DOS ('Sigma Four', a MS-DOS clone); OTS ('One Track System', an OS for 128KB systems). Base system shipped software on cassettes.