Peritek
5-18
Programming On-board Devices and Memories
This section contains three parts:
a) a discussion of the VMEbus and 34020 byte ordering issues
b) example code for software byte swapping
c) a discussion of the new hardware byte swapper
5.3.5a VMEbus and 34020 Byte Order Mapping
The VMEbus observes reversed byte polarity relative to the 34020 and the
board internal bus. It considers its byte 0 (A0/A1 = 00) to be the high byte,
data bits 24-31, while its byte 3 (A0/A1 = 11) is the low byte, data bits 0-
7. Since the VMEbus can do byte and word operations on board memories,
care must be taken that bytes are not inadvertently swapped.
Long word operands are stored by the VMEbus with the high order word
(bytes 0 and 1) preceding the low order word (bytes 2 and 3) in memory,
the reverse of the 34020. The TI cross assembler/compiler tools will
supply the object code in big or little endian. The Peritek downloader and
software are written for little endian. The VMEbus must use an
odd
address for byte access of 8-bit devices (color maps, cursors, DUART,
DLUT).
Table 5-8 Byte/Word/Longword Mapping
VME
34020 Byte
Data
Address
Address
Lines
Little Endian Byte Addressing
0
3
24 - 31
1
2
16 - 23
2
1
8 - 15
3
0
0 - 7
Big Endian Byte Addressing
0
0
24 - 31
1
1
16 - 23
2
2
8 - 15
3
3
0 - 7
Little Endian Word Addressing
0
2
16 - 31
2
0
0 - 15
Big Endian Word Addressing
0
0
16 - 31
2
2
0 - 15
Big or Little Endian Long Addressing
0
0
0 - 31
Содержание VCD-V
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Страница 41: ...Peritek 2 10 Installing Your Peritek Graphics Board Figure 2 2 Jumper Locations for the VCT V and VCU V ...
Страница 42: ...Peritek Installing Your Peritek Graphics Board 2 11 Figure 2 3 Jumper Locations for the VCD V ...
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Страница 95: ...Peritek 4 16 Theory of Operation Figure 4 2 VCU V Block Diagram ...
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