address in the monitor to end a program. This will cause
problems. It is always safe to jump to address 0, which
restarts the monitor program correctly.
If you don’t want to clear the screen
Reset the stack to 0C33H
then jump to PARSE
Notes on PIO Operation.
The Nascom I has on board two totally uncommitted 8 bit
parallel I/O ports complete with handshake lines, in the
shape of an MK3881 Z80 – PIO. ‘The PIO is, in itself, a
fairly complicated processor, which needs programming
before it will operate in any of its 4 modes:
|INPUT||MODE 1||(automatically set on PIO Reset)|
It is not the purpose of these notes to describe in detail
these operational modes, but to help clear up a few
common problems encountered in controlling the PIO.
One very important fact to note is that the PIO is not
reset by the RESET button on the keyboard. This resets
the CPU only, NOT the PIO. It may be reset in two ways.
The simplest is to switch the power off and on again; a
bit drastic but the PIO does have automatic power on reset.
The second method (shown in fig.3 ) is to apply an M1
without either RDor IORQ. It should, however, be pointed
out that, since the CPU can be reset, it is always possible
to regain control of the PIO in software, by simply
Now to ‘interrupts’. Don’t forget that the PIO is designed
to operate in the Z-80 Interrupt Mode 2, so before doing
anything put the CPU into this mode by executing ‘IM 2’
(HEX code ED 5E). Remember that a CPU reset puts the Z-80
back to Interrupt Mode 0 clears the I register, and dissables
CPU interrupts (having no effect on the PIO).
In Interrupt mode 2, the CPU finds the address of the
interrupt routine, by loading the Program Counter (P.C.)
with the contents of the memory address. This is formed
by the I register (high byte), and the interrupt vector
sent from the interrupting port (low byte).
/ .. ..