Intel 80C186XL, 80C188XL user manual A191 D158 BHE Low D70 A0 Low, Bit Data Bus Even Word Transfers

BUS INTERFACE UNIT

(X + 1)

A19:1	D15:8 BHE
	(Low)

(X)

D7:0 A0 (Low)

A1107-0A

Figure 3-3. 16-Bit Data Bus Even Word Transfers

During a byte read operation, the BIU floats the entire 16-bit data bus, even though the transfer occurs on only one half of the bus. This action simplifies the decoding requirements for read-only devices (e.g., ROM, EPROM, Flash). During the byte read, an external device can drive both halves of the bus, and the BIU automatically accesses the correct half. During the byte write op- eration, the BIU drives both halves of the bus. Information on the half of the bus not involved in the transfer is indeterminate. This action requires that the appropriate bank (defined by BHE or A0 high) be disabled to prevent destroying data.

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