TC1796
System Units (Vol. 1 of 2)
Data Memory Unit
User’s Manual
8-7
V2.0, 2007-07
DMU, V2.0
8.5.1
Internal Overlay
For internal overlay, the size of the blocks can be 2
n
, with n = 1 to 9 (2 to 512 byte). Thus
the maximum memory size needed for overlay memory (if all blocks are used with
maximum size) is 8 Kbyte. Only the 64 Kbyte DMU SRAM (not the Stand-by SBSRAM)
can be used as internal overlay memory.
During address translation, the segment number is set to C
H
.
8.5.2
Emulation Memory Overlay
For Emulation Memory Overlay (only available in the emulation device TC1796ED), the
size of the blocks can be 2
n
×
1 Kbyte, with n = 1 to 5 (2 Kbyte to 32 Kbyte). Thus, the
maximum memory size needed for overlay memory (if all blocks are used with maximum
size) is 512 Kbyte.
8.5.3
Switching between Internal and Emulation Memory Overlay
When switching a region between internal and external overlay, it has to be ensured that
the respective OVEN bit is cleared, all region parameters are set properly, and then the
region is enabled again. Other wise, unintended access re-directions may occur.
8.5.4
Region Priority
If concurrent matches in more than one region occur, the region with the lowest order
number will win and perform the address translation.
8.5.5
Access Performance
Write Accesses
Write accesses are performed with zero wait states.
If no overlay is enabled, the accesses to the DMU memories take one cycle (1 wait
state). If a pipelined read follows immediately a write, the read will be delayed by an
additional wait state.
If an overlay is enabled, the access to the DMU overlay memory block may be prolonged
by one cycle.
The access to the Emulation Memory will not be more than 10 cycles. Details can be
found in TC1796ED specific documents.