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30 lines
1.4 KiB
Plaintext
30 lines
1.4 KiB
Plaintext
=============================================================================
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AST RESEARCH, INC. TECHNICAL BULLETIN #0629 06-27-90
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=============================================================================
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AUTOSENSING OPERATION OF THE VGA PLUS
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When the AST VGA Plus (part number 202262-003) is configured to operate in 16
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bit mode (switch position "two" = ON) the entire memory address range from
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C0000h to DFFFFh is in 16 bit mode for all system memory accesses. This is
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due to the design of the IBM AT standard I/O bus. This means that an 8 bit
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adapter which is configured to reside in this region will not be recognized
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by the system.
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Version 3.0 of the VGA Plus BIOS (part numbers 107000-682 and 107000-683) has
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the ability to detect the standard IBM ROM signature "AA 55". If, when the
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system is being initialized, the VGA BIOS detects this signature anywhere in
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the 128K range where it resides, the VGA Plus will switch back to eight bit
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operation to avoid conflicts.
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The BIOS cannot detect the presence of RAM (e.g., shared memory) on an
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adapter, however. In order for adapters which have this type of memory
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(e.g., IBM Token Ring II adapters) to be configured to operate in this
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address range, the VGA Plus has to be manually switched back to 8 bit
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operation (switch position "two" = OFF) to avoid conflicts.
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