Motorola M68705EVM User Manual page 74

4.2.1.1 Map Switching. The EVM operates in either one of two memory maps (monitor
or user map) as illustrated in Figure 4-2. Two types of memory map switching (temporary or
permanent) can be performed. Temporary map switching allows modification of user
memory, and permanent map switching allows execution of user programs.
Temporary map switching is used to modify user pseudo ROM (RAM). The opcode and
operand are fetched from the monitor map, memory maps are switched for one cycle, and the
read or write cycle is executed in user space. Memory maps are then automatically switched
back to the monitor map on the next cycle.
Permanent map switching is initiated by a command from the monitor, or by the USER
RESET switch S1. The return from interrupt (RTI) opcode is fetched from the monitor map,
memory maps are switched, and the user register contents are fetched from the user stack.
The Program Counter (PC) content is fetched from the user stack and execution proceeds
from the current PC value.
When the USER RESET switch S1 is activated (pressed), the MCU and user 1/0 ports are
reset, memory maps are switched to the user map, and the reset vector is fetched from the
user map by the MCU. Breakpoints are ignored during this operation.
Execution of user code continues until a software interrupt (SWI) is decoded on the data bus
during a Load Instruction Register (UR) cycle. SWI occurs when either a breakpoint is
detected, or the ABORT switch S2 is activated. The memory map is switched back to the
monitor map after the user register contents are saved on the user stack. An SWI which
occurs when no breakpoint is set -- or when the ABORT switch is not activated -- does not
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memory map switching to take place. This allows user SWls to be executed in real
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The ABORT switch, when activated, forces an SWI on the data bus until the next UR cycle.
The user register contents are saved on the user stack, and memory maps are switched to
the monitor map. The ABORT switch has no effect on the monitor map. When the ABORT
switch is activated while in the user map, the monitor map is re-entered (assuming the MCU
is operating properly in the user map).
EVALUATION MODULE USER'S MANUAL
4-2
MOTOROLA
M68705EVM