Attemtping to get off the ground, I wrote an
“IPL” which, instead of loading anything will
simply signal the serial line indicating that
it has been successfully executed.
So far, no luck.
I have examined the binary, and have seen
that the jump instruction put 16 bytes below
the top of the EPROM I am using for the pseudo-IPL
is a 16 bit relative jump with a value of 0x7fed.
This has to be a negative number which tells me
that the value in the jmp instructions should have
a “1” in the left-most bit position to represent
a signed, 16-bit quantity. This does not. (The
location to which the jump should go is 0x17fe0
above the bottom of the EPROM.) How does the program
counter “know” that it is supposed to take this (0x7fed)
as a negative number?
I am not SURE this is the problem, but I
am curious to know how this is supposed to work.
In case you are wondering,
I did try substituting an absolute jump
(EA EO 7F 00 F0), but still had no luck getting
any sign of life on the serial line.
Thank you.
Richard Bonomo
bonomo@sal.wisc.edu