Rabbit 2000 - Page 16

12 Rabbit 2000 Microprocessor
• Extended Code—Instructions not in root that often require 20-bit addressing for access.
These are accessed via the extended code segment, which is an 8K page for executing
code. Up to a megabyte of code can be executed by moving the mapping of the 8K
window using special instructions (long call, long jump and long return) that are
designed for this purpose.
• Extended Constants—Constant data not in root that requires 20-bit addressing for
access. This is mixed together with the extended code.
• Extended Bulk Memory—Data items stored in multimegabyte memory and accessed
using special functions using 32-bit addressing.
Code may be placed in either extended memory or root memory. Code executes slightly
more efficiently in root memory. In large programs the bulk of code is stored in extended
code memory. Since root constants and root variables share the memory, space needed for
root code, and as the memory needed for constants or variables increases, the amount of
code that can be stored in root must decline by moving code to extended memory. The rel-
ative size of the root and data segments can be adjusted in 4K steps.
Figure 2. Schematic Map of 16-bit Addressing Space
It should be clear from the above Dynamic C memory types that the Rabbit does not have
a “flat” memory space. The advantage of the Rabbit’s memory organization is that the use
of 16-bit addresses and pointers is retained, ensuring that the code is compact and executes
quickly.
4.1.1 The Root (or Base) Segment
The root segment has a typical minimum size of 8K and a maximum size of 48K. The
larger the root segment, the smaller the data segment and vice-versa. Root segment
address zero is always mapped to 20-bit address zero. Usually the root segment is mapped
64K
56k
52K
0k
Extended Code
Segment
Stack Segment
Data Segment
Root Segment
1-megabyte
Typical mapping
16-bit to 20-bit address space
RAM
Flash
quadrant 0
quadrant 3
quadrant 1
quadrant 2
flash
RAM
RAM