(h) SOFTWARE –DISC OPERATING SYSTEM
NASCOM have announced their intention to provide the CP/M
operating system, for disc users. This will open up the
possibility of running hundreds of CP/M programs which already
exists, including larger more powerful versions of BASIC, as
well as other languages. We hope you will support the
committee’s belief that this is the way that we should go
for large systems.
We hope that we have answered all the questions asked by
Mr.Griffiths. If not, write back to us. All queries are
welcome, although we can never hope to answer all of them.
I thought NASCOM users might be interested in some additional
Z80 opcodes that I have discovered. They all operate on
IX or IY and their effectiveness hinges on the fact that
IX and IY are functioning similar to HL. In general,
instructions operating on HL will operate on IX or IY if
preceeded by DD or FD. From this it can be deducted that the
internal microcode of the Z80 addresses HL, IX, IY indirectly
by a 2 bit register address pointer which is cleared at the
start of each new instructions. When this pointer (call it P)
is 00 the instructions operate on HL when it is 10 they
operate on IX and when it is 11 they operate on IY. The
effect of the instruction prefix DD is to set P to 10 and
the effect of FD to set it to 11. Similarly, there must be
other flip flops to select between the alternate register
and accumulator sets.
Because of this any instruction normally accessing the
H or L registers can be used to access the high or low order
bytes of IX or IY if prefixed by DD or FD. This gives 80
new instructions. I have tested these on a NASCOM using a
for rotate and shift, test, set, reset, which would give
124 more instructions. I have tested EX DE and IY and found
that they do not work.