80-Bus News

Introduction

I hear so much these days about the advantages of having a 16 or even 32 bit computer that I start becomming depressed when sitting down to my humble 8 bitter. However, with so much Z80 software I have a made considerable investment which I can’t throw away lightly. So now the Z80 fights back. I’m not going to fit a Z80B because I don’t know if the peripherals will support it and I don’t know how to get the 6 MHz clock rate required.

Instead I am going to use interrupts to handle some of the devices. The first is going to be the printer which is after all attached to a PIO making the whole thing pretty simple.

The Hardware

My humble 8 bitter is a Gemini Galaxy 2 running CP/M 2.2 with an Epson FX80 connected via the PIO using the Centronics protocol. The printer is reputed to have a 2K internal buffer but this doesn’t seem to make printing a smooth operation. No hardware changes are required except to the IVC as described in 80-BUS News Vol 2 Issue 3. (If the system has an SVC then no changes are required at all.)

The Theory

The new printer driving routines are built around a 256 byte buffer which with single byte pointers provides automatic wrap around making the buffer appear circular. There is an input pointer which points to the next free entry for the next character to be printed and an output pointer which points to the next character to by output to the printer. The buffer is empty when the input pointer equals the output pointer and it is full when the input pointer is one behind the output pointer (i.e. the input pointer has gone right round and caught up with the output pointer). The printer generates an interrupt when busy goes low provided that interrupts are enabled.

The Software

The full listing is somewhere else in the magazine and shows a test program which illustrates the theory. The routine labels correspond to those in the Gemini BIOS (section 3.6 of The Galaxy Computer User’s Manual) where applicable and these routines are intended to replace the existing ones. INIT has to replace the existing printer initialisation and the interrupt service routine must also live somewhere in the BIOS.

Where to locate the interrupt vector is a bit of a problem. Wherever it goes defines the 256 byte page of interrupt vectors and for this reason I have put it in the BIOS scratch area in page zero but people with 64K RAM boards used as a RAM disk will find the inter-page transfer code there.

Finally, the biggest problem. Accessing the floppy disk drives is very time critical and they will lose data if interrupted during data transfers so it is necessary to find some way of disabling and re-enabling interrupts around disk accesses. I think that a couple of NOP instructions strategically placed in the disk driver routines would have been the answer, as they could be patched to DI and EI if required. This addition would also make it possible for users to attempt multi-tasking.

IVC Mods

The first thing I noticed when the code started to run was that the IVC was being reset every so often. I found the answer in Vol 2 Issue 3 of 80-BUS News and I am sure no-one will mind if I repeat it here.