The NAS-SYS Monitors

By J. Haigh

Single Step S xxxx

The Single Step command initially uses part of the machine code used by the Execute command, described in article 2. It enters the Execute routine at the point at which it throws away the return address by POPing it into AF; it thus misses out the section which sets the workspace byte CONFLG (£0C26) to −1, and this remains at zero. Continuing with the Execute machine code routine, the Single Step command saves the specified start address in the monitor workspace, loads registers BC, DE, HL, AF and SP from the register save area in the workspace, and pushes the start address onto the top of the stack. The AF registers are then saved while bit 3 of port 0 is set; this activates a TTL circuit which sends a non-maskable interrupt to the processor after four M1 cycles. The AF registers are recovered, and this is followed by a return-from NMI instruction (RETN, ED 45), which, because the start address was pushed on the top of the stack, causes the program to start executing at this address. Three M1 cycles have now occurred (one each time a byte is fetched from memory), so as soon as the next instruction is started the NMI line is activated, and the processor is interrupted at the end of this instruction.

This causes the processor to jump to the NMI handling routine. Here bit 3 of port 0 is reset, and the value stored at CONFLG is tested. If the value is not zero the program must have arrived at this point from an Execute command, and the routine continues as described in the second article. If CONFLG is zero the top 10 bytes of the user stack, which contain the value of the program counter register for the next instruction of the program being single-stepped (pushed on the stack as the return address by the NMI) and the AF, HL, DE and BC registers, are copied to the register save area in the monitor workspace. The contents of these registers, together with the user stack pointer, the interrupt register and the index registers IX and IY, are then printed on the screen.

In Nas-Sys 1 the routine which prints the registers is part of the NMI/​Breakpoint handling routine, but in Nas-Sys3 it is written as a separate subroutine which can be accessed by command P or called from other programs. The format of the register print out is also different in the two monitors. In Nas-Sys 1 only the registers are printed out, in the order:–

SP     PC     AF     HL     DE     BC     I     IX     IY

followed by a string of letters indicating which of the bits in the flag register are set. In Nas-Sys 3 the display of each of the first six register pairs is followed by the sixteen bit value held in the memory location to which the register pair points. Thus if the H