80-Bus News

Having got these two exceptions out of the way, let us now look at the effect of an NMI. When the CPU recognises that the NMI line is active, it immediately saves the address of the current instruction, performing a call to location 0066H. Here, in theory, is the service subroutine for the NMI interrupts. This service routine should do whatever is required on an NMI and finish with a RETN (RET from NMI) instruction. This RETN instruction behaves like an ordinary RET instruction in popping the return address off the stack, but also acts as a signal to the CPU that the NMI routine is finished, and that the previous status of the normal interrupt structure can be restored. An NMI will automatically disable any other interrupts, saving the status until its own service routine is finished. So, if interrupts were enabled before an NMI, they are enabled afterwards, and contrariwise.

On the Nascom, the NMI is used for a specific purpose, well known to those of us who are into machine code. This is to provide a Single Step facility. (On Gemini systems the NMI is uncommitted, and Single Stepping is provided in software by Gemdebug.) The Nascom Single Step works as follows: on Execution of a command, a bit is set in Port 0. This bit is clocked through a succession of flipflops, ending up toggling the /NMI pin of the CPU. The CPU immediately jumps to the service routine, which looks to see if the instruction executed was an E or S. If E, the CPU carries on with the next instruction, but if S, it prints the register display.

As the Nascom has ROM at location 0066H (unless it is set up to run CP/M), you might assume that other use of the NMI is out of the question for Nascom users. Not so! At 0066H is a jump to 0C7DH, and this address is in RAM, so we can patch in here the address of our NMI handler. N2 owners can connect an NMI switch to their machine, which triggers a monostable to give a nice clean pulse, normally causing the register display to be printed out. N1 users can get this with a little modification published in INMC No.2. Its use is for those occasions when a program locks in a loop due to a programming flaw. Hitting NMISW can find where this loop is, and speed up debugging. N2 owners can also connect an NMI Signal to Bus line 21. Zilog suggest that the NMI be used for catastrophic events, such as a power failure, where big capacitors on the PSU lines could maintain power for a short time after mains failure, long enough to save the data to disk perhaps. Using it to provide a single step facility will be regarded by most Nascom Users as quite satisfactory, but I deal with the NMI specifically because it is a free interrupt handling method, and when using the Z80 as the basis for an intelligent controller of some sort, it might provide an elegant simplification of the unit. To give you an idea of the potential power of the NMI, it is the method by which the IMP printer is driven. Within the IMP, an NMI is caused at regular intervals. The internal CPU (a Z80 – what else!) switches the address of the NMI routine around between a number of routines to look after checking for new data and printing each column of dots.

Now we move on to the ‘normal’ interrupts. There are three schemes. These are called Interrupt Mode 0, 1 and 2, and invoked by instructions IM n, where n is the appropriate number.

Interrupt Mode 0 is identical to the 8080 interrupt response mode. At the end of each instruction, the CPU looks at /INT (pin 16). If this line has gone low, something has requested an interrupt. Note that this line need not be low when examined. Its status is internally latched and reset when the CPU reads it. The CPU acknowledges this if interrupts have been enabled by making /M1 and /IORQ lines go low. The interrupting device is supposed to recognise this signal and to place the next instruction to be executed on the bus. Normally this instruction is a restart, which is a call to one of 8 specific locations, but it can be an actual three byte call if you wish, and if you can sort out how to do it! While acknowledging the interrupt, the CPU executes two wait states to allow time for the peripheral devices to sort out which one has priority. When it accepts an