MC68HC812A4 Datasheet, PDF (83/342 Pages) Motorola, Inc – 16-bit device composed of standard on-chip peripheral modules connected by an intermodule bus. Modules include

Operating Modes and Resource Mapping
Internal Resource Mapping
5.3.3 Background Debug Mode
Background debug mode (BDM) is an auxiliary operating mode that is
used for system development. BDM is implemented in on-chip hardware
and provides a full set of debug operations. Some BDM commands can
be executed while the CPU is operating normally. Other BDM
commands are firmware based and require the BDM firmware to be
enabled and active for execution.
In special single-chip mode, BDM is enabled and active immediately out
of reset. BDM is available in all other operating modes, but must be
enabled before it can be activated. BDM should not be used in special
peripheral mode because of potential bus conflicts.
Once enabled, background mode can be made active by a serial
command sent via the BKGD pin or execution of a CPU12 BGND
instruction. While background mode is active, the CPU can interpret
special debugging commands, and read and write CPU registers,
peripheral registers, and locations in memory.
While BDM is active, the CPU executes code located in a small on-chip
ROM mapped to addresses $FF20 to $FFFF, and BDM control registers
are accessible at addresses $FF00 to $FF06. The BDM ROM replaces
the regular system vectors while BDM is active. While BDM is active, the
user memory from $FF00 to $FFFF is not in the map except through
serial BDM commands.
5.4 Internal Resource Mapping
The internal register block, RAM, and EEPROM have default locations
within the 64-Kbyte standard address space but may be reassigned to
other locations during program execution by setting bits in mapping
registers INITRG, INITRM, and INITEE. During normal operating
modes, these registers can be written once. It is advisable to explicitly
establish these resource locations during the initialization phase of
program execution, even if default values are chosen, to protect the
registers from inadvertent modification later.
Advance Information
MOTOROLA
Operating Modes and Resource Mapping
MC68HC812A4 — Rev. 3.0
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