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NAME
- WriteExpansionByte
-
write a byte nybble by nybble.
SYNOPSIS
WriteExpansionByte( board, offset, byte )
A0 D0 D1
FUNCTION
(Not typically called by user code)
WriteExpansionByte writes a byte to a new-style expansion board. These boards have their writeable data organized as a series of nybbles in memory. This routine writes two nybbles in a very careful manner to work with all types of new expansion boards.
To make certain types of board less expensive, an expansion board's write registers may be organized as either a byte-wide or nybble-wide register. If it is nybble-wide then it must latch the less significant nybble until the more significant nybble is written. This allows the following algorithm to work with either type of board:
WriteExpansionByte writes a byte to a new-style expansion board. These boards have their writeable data organized as a series of nybbles in memory. This routine writes two nybbles in a very careful manner to work with all types of new expansion boards.
To make certain types of board less expensive, an expansion board's write registers may be organized as either a byte-wide or nybble-wide register. If it is nybble-wide then it must latch the less significant nybble until the more significant nybble is written. This allows the following algorithm to work with either type of board:
write the low order nybble to bits D15-D12 of
The offset is a byte offset into a ExpansionRom structure. The actual memory address read will be four times larger. The macros EROFFSET and ECOFFSET are provided to help get these offsets from C.
byte (offset*4)+2
write the entire byte to bits D15-D8 of
byte (offset*4)
INPUTS
- board
-
a pointer to the base of a new style expansion board.
- offset
-
a logical offset from the configdev base
- byte
-
the byte of data to be written to the expansion board.
EXAMPLE
WriteExpansionByte( cd->BoardAddr, ECOFFSET( ec_Shutup ), 0 );
WriteExpansionByte( cd->BoardAddr, ECOFFSET( ec_Interrupt ), 1 );
WriteExpansionByte( cd->BoardAddr, ECOFFSET( ec_Interrupt ), 1 );