Dataman S4 - Manual (Page 44)

DATAMAN S4 MANUAL
44 EMULATE KEY
Memory-Emulator will do most things that an
Microprocessor Development System can do,
and most MDSs do not have editors and code
manipulators which are as powerful as those of
S4. S4 will not do anything microprocessor-
specific – but it is not usually too difficult to
write a routine in the code of the target-
microprocessor, whereby a subroutine-call or
software-interrupt may be substituted for any
instruction, and the microprocessors registers
dumped to a specific area of USER RAM
where they can be inspected. The
microprocessor in the target system can
WRITE as well as READ. It true that PROMS
do not have a WRITE input, but that is
provided separately on a flying lead.
Microsystem memory selection
In a microsystem, ROM and RAM are selected
by decoding the address bus to derive unique
CHIP SELECT signals. Each chip has its own
slot in addressing space. ROM and RAM
output their data on the system bus when their
address is selected and the microprocessor's
READ line becomes true: the READ line
should be connected to the OUTPUT ENABLE
lines of all chips in the system. The
microprocessor sees no difference between
ROM and RAM, when reading. During a
WRITE cycle the READ line stays false and no
memory chip can output data on the bus. This
prevents conflict with the data written by the
microprocessor. The WRITE output of the
micro is connected to the WRITE input of the
RAM. When the WRITE signal becomes true
the write-data output by the microprocessor is