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TC1796
System Units (Vol. 1 of 2)
Data Memory Unit
User’s Manual
8-6
V2.0, 2007-07
DMU, V2.0
The size of the overlay memory blocks can be 2
n
times the minimal block size (2 byte or
1 Kbyte). The start address of the block can be a integer multiple of the programmed
block size (natural page boundary).
If the overlay block is enabled and the segment address is 8
H
or A
H
, the segment offset
of the target address is compared with the target block address of the overlay target
address. This bit-wise comparison is qualified by the content of the mask, ignoring the
address bits that form the offset into the overlayed block.
If there is no match, the original target address is taken to perform the access.
If there is a match:
•
The most significant part of the segment offset, that addresses memory sizes beyond
the maximum overlay memory size (64 Kbyte for DMU, 1024 Kbyte for TC1796ED
emulation memory), is set to predefined values according to the address map (fixed
bits in registers RABRx).
•
The part of the target block address, that corresponds to the overlay block base
address and is masked, is replaced by the respective overlay block base address bits
(bits OBASE in RABRx, where the corresponding mask bits OMASK in registers
OMASKx are set to “1”).
•
The address is completed by the original offset into the block; the number of bits used
are determined by the bits set to “0” in the mask OMASK.
Minimum Block Sizes for DLMB Transactions
The overlay functionality provides the possibility to redirect data accesses with block
sizes of 2 bytes or higher. The redirection is done by a modification of the target address
as shown in
. If the data size of a DLMB transaction in TC1796 is greater than
the selected overlay memory block size, the following behavior will be observed:
•
If the address of the DLMB transaction matches the overlay target address, the
complete transaction is redirected, and the complete data (exceeding the block size)
is taken from the overlay memory.
•
If the address of the DLMB transaction does not match the overlay target address,
the transaction is not redirected even if the overlay block resides within the
transaction data block.
Therefore, it is recommended not to use DLMB transactions which have a greater data
size as the programmed minimum overlay memory block size (OMASK bit field in
Register OMASKx). This recommendation can, for example, be guaranteed by using a
minimum overlay memory block size of 8 byte.