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is installed, all memory cycles are inhibited whenever the
backplane signal PHANTOM (pin 67) is low.
some systems to superimpose a ROM over areas normally occupied by
RAM.
BANK SWITCHING
Provision is made for a system with more than 64K bytes of memory
by allowing more than one board to occupy a given address region,
as long as only one board responds to any memory reference.
is accomplished by the flip-flop OCCLUDE which, if set, makes the
entire board ' ■invisible" to the processor.
The OCCLUDE flip-flop may be set or reset by an OUT instruction
to port C0 hex.
changed by rearranging the address inputs to the board.
0 cioes to the D input of this flip-flop.
flop may be gated by any of the other seven data bits (selection
made by jumper).
"banks" of 64K, or 28 boards.
The CLR signal can initialize OCCLUDE either ON or OFF.
PARITY
The PARITY-ARM flip-flop is programmed in the same manner as
OCCLUDE:
its D input comes from data bit 0 and its clock can be
gated via a jumper by any of the other seven data bits.
pulse that clocks PARITY-ARM on its trailing edge clears the
PARITY-ERROR flip-flop.
PARITY-ERROR is set during a memory fetch cycle if a byte with
even parity is read and, via a driver, lights the on-board LED.
The AND of PARITY-ARM and PARITY-ERROR goes to another driver,
the output of which the user may wire to one of the eight
vectored interrupts, to PINT, or to N M I .
The CLR signal initializes the PARITY-ERROR and PARITY-ARM flip-
flops off.
The port number is hardwired and may only be
Thus, we have a theoretical maximum of seven
North Star 16K RAM Board
PHANTOM is used in
Data bit
The clock to the flip-
The same
This
30
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