The STM8 is very similar to the earlier ST7, but is better suited as a target for C due to its 16-bit index registers and stack pointer-relative addressing mode. Although internally a Harvard architecture it has a unified 24-bit address space, allowing code to execute out of RAM (useful for in-system programming of the flash ROM), and data (such as lookup tables) to be accessed out of ROM. Access to data above 64K is limited to special “load far” instructions; most operations' memory operands can access at most 128K (a 16-bit base address plus 16-bit offset).

Very few STM8 microcontrollers have more than 64K of data memory.

It has the same six registers (A, X, Y, SP, PC, CC) as the ST7, but the index registers X and Y have been expanded to 16 bits, and the program counter has been expanded to 24 bits. The accumulator A and the stack pointer remain 8 and 16 bits, respectively.[2]

The condition code register has two more defined bits, for a total of seven. There is an overflow flag, and a second interrupt enable bit, allowing four interrupt priority levels.