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Rabbit 4000 Microprocessor User’s Manual
5.3 Operation
5.3.1 Memory Management Unit (MMU)
Code execution takes place in the 64KB logical memory space, which is divided into four
segments: root, data, stack, and extended (XMEM). The root segment is always mapped
starting at physical address 0x000000, but the other segments can be remapped to start at
any physical 4KB block boundary.
The data and stack segment mappings are set by writing to the appropriate register, as
shown in Table 5-1. The DATASEG and STACKSEG registers provide backwards
compatibility to the Rabbit 2000 and 3000 processors; these registers map directly to
DATASEGL and STACKSEGL but the corresponding uppermost four bits are set to zero.
Each of these registers provides a 4KB offset that is added to the logical address to pro-
vide a physical address as shown in Figure 5-3.
Table 5-1. Memory Management Registers
Register
Segment
Size
Comments
DATASEG
Data
8 bits
Maps to DATASEGL;
DATASEGH set to 0x00
DATASEGL
Data
8 bits
—
DATASEGH
Data
4 bits
—
STACKSEG
Stack
8 bits
Maps to STACKSEGL;
STACKSEGH set to 0x00
STACKSEGL
Stack
8 bits
—
STACKSEGH
Stack
4 bits
—
XPC
XMEM
8 bits
Loaded via instructions
LD XPC,A
and
LD A,XPC
LXPC
XMEM
12 bits
Loaded via instructions:
LD LXPC,HL
and
LD HL,LXPC
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