Peritek
Programming On-board Devices and Memories
5-17
5.3.4 Memory Types and Sizes
The board has a maximum of 8 MB (VCD-V) or 16 MB (VCT-V and
VCU-V) of display memory and 32 MB of 34020 system memory capacity
contained on field replaceable Single Inline Memory Modules (SIMMs).
In addtion, four 32-pin PLCC sockets are provided for 8-bit Flash
EEPROM devices (see Sections 2.4.4 and 5.17 for more information). The
memory possible combinations are manifold and are designed to provide
the experienced user with memory tailored to the application. Please
contact Peritek to discuss your requirements.
The 34020 shares access to the display and system memories with the
VMEbus by means of the 34020's host interface. When accessed by the
VMEbus or 34020, the display and 34020 (non-EPROM) memories are
byte addressable.
The board memories can be accessed by the VMEbus in one of two ways:
A 1 KB "window" in A16 or A24 space which gives access to memory
selected by the LAR (see Section 5.2) or a 64 MB block in A32 space.
When used, the hardware byte-swapper quadruples these memory sizes.
The 34020 addresses long (32-bit) words, so masking must be done to
limit the operation to a single byte or less (if desired). The masking may be
done by setting the proper bit field size in the 34020, or, if the operation is
to be performed on the display memory, by using the writemask register
(see Section 5.3.3).
5.3.5 Byte Ordering and the Hardware Byte Swapper
The infamous big endian/little endian dilemma must be dealt with here.
Big endian means that the least significant byte (Byte 0) is assigned to the
high order (DA24-DA31) data lines. Conversely, little endian means that
the least significant byte (Byte 0) is assigned to the low order (DA0-DA7)
data lines. The merits of one way or the other is truly religious, and
therefore we won't talk about that. But we do have deal with the
consequences.
The 34020 is intrinsically a little endian device. The VMEbus is big
endian. Although the 34020
does have
a big endian mode there is a
performance penalty for using it. Therefore, the graphics board is designed
to run little endian. A programmable byte swapper is included to alleviate
some of the CPU overhead which is otherwise incurred.
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