ROM DISABLE (ORG 0)
The ROM disable function is normally used with the
CP/M operating system. Since the system RAM must
be a continuous block originating at zero instead of
8k, your first RAM circuit board will be addressed at
000 instead of 040.
You can also use this function with Microsoft Basic
under HDOS, since the unused memory area between
PAM-8 and the Hl7 ROM/RAM is an excellent loca
tion for user functions. You must take care in this
application, however, not to overwrite the PAM-8 or
Hl7 ROM/RAM areas. This is due to the fact that these
areas now exist in read/write RAM, rather than ROM
and write-protected RAM. You can also use up to 64 k
of memory with this configuration.
NOTE: If you have less than 64 k of memory, you can
use HDOS Version 1.6. For 64 k of memory, you must
use HDOS Version 2.0.
This function pulls the ROM disable line on the bus
(pin 46) low when a write is performed at I/O address
362Q (0F2H) and it sets data bit D5 to a logic 1. This,
in turn, disables the ROM on the CPU circuit board
and allows you to address the RAM between 0 and 8 k
in the memory space.
To use the ROM disable function, make the following
modifications to the circuit boards in your H8 compu
ter.
( ) Remove the tie bracket from your Computer.
( ) Carefully unplug the H17 controller circuit
board from your Computer.
( ) Check the controller circuit board to see if there
is an IC U28 (in the upper left corner of the
board). If there is, disregard the following in
formation and proceed to Page 25. If there is no
IC at this location, perform the following three
steps.
1.
( ) Refer to Pictorial 2-3 below and carefully
remove IC U22 from the circuit board.
2.
( ) Refer to Detail 2-3A, position the IC as
shown, then bend pins 2 and 13 up so they
will not make contact when the IC is
reinstalled.
3.
( ) Reinstall the IC at U22. Make sure you posi
tion the pin 1 end toward the index mark on
the circuit board.
Detail 2-3A
PICTORIAL 2-3
