Step 3. Reprogram Chip-Select Base Addresses - HP 64746 User Manual

Chapter 5: Plugging into a Target System
Configuring the Emulator for In-Circuit Operation

Step 3. Reprogram chip-select base addresses

If you are not going to be changing the base address of chip-select 0 from address
0, or changing the base address registers of the other chip-selects after initial setup,
you should skip this step. Dealing with chip-selects whose base addresses are
being changed is probably the most difficult challenge you will face with the 68302
emulator.
Before you can successfully emulate in this mode, it is important that you
understand how the 68302 uses chip-selects. When you enter memory map terms
in your configuration, you will notice that you enter all address ranges without the
option to qualify a specific range with a chip-select number. What this means is
that the emulation memory mapper cannot "track" changes made to the base
address of any chip-selects. Any emulation memory ranges you map respond based
on the value on the address bus (and optionally the function code). The chip-select
signals are not used by the emulator to decode memory map terms.
What does this mean? This means that if you are going to be mapping a block of
memory to either emulation RAM or emulation ROM and are going to be changing
the base address of that block, you must ensure that valid code or data exists at
BOTH blocks of memory. For example, if you have boot code that originally exists
at address 0 and chip-select 0 is later reprogrammed with a base address of
400000H you must ensure that the code exists at both 0 and at 400000H. If you are
executing your code from target memory, this is easy, since your target hardware
will be designed to decode memory based on chip-select 0 being active, not based
on the full address (A23-A1).
Here is an example of how you can emulate a setup where the chip-select base
addresses are being changed. The technique of switching the RAM and ROM
addresses closely follows an example that Motorola provides in Revision 2 of the
MC68302 User's Manual.
We start with the following memory map:
000000H thru 00FFFFH emulation ram
400000H thru 41FFFFH emulation ram
800000H thru 800FFFH target RAM
and use the following program:
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