Separate stack space

Separate stack space

One of the bits in the processor state word (see below) was indicating that the processor is accessing data from the stack. Using this signal, it was possible to implement a separate stack memory space. However this feature was seldom used.

The internal state word

For more advanced systems, during one phase of its working loop the processor set its "internal state byte" on the data bus. This byte contains flags which indicate whether the memory or I/O port is accessed and whether it was necessary to handle an interrupt.

The interrupt system state (enabled or disabled) was also output on a separate pin. For simple systems, where the interrupts were not used, it is possible to find cases where this pin is used as an additional single-bit output port (the popular Radio86RK computer made in USSR, for instance).