Intel 80C186XL, 80C188XL user manual MByte, KBytes

BUS INTERFACE UNIT

Physical Implementation			Physical Implementation
of the Address Space for			of the Address Space for
	8-Bit Systems			16-Bit Systems
	1 MByte		512 KBytes		512 KBytes
	FFFFF		FFFFF		FFFFE
	FFFFE		FFFFD		FFFFC
	2		5		4
	1		3		2
	0		1		0
A19:0	D7:0	A19:1	D15:8	BHE	D7:0	A0
						A1100-0A

Figure 3-1. Physical Data Bus Models

Byte transfers to even addresses transfer information over the lower half of the data bus (see Fig- ure 3-2). A0 low enables the lower bank, while BHE high disables the upper bank. The data value from the upper bank is ignored during a bus read cycle. BHE high prevents a write operation from destroying data in the upper bank.

Byte transfers to odd addresses transfer information over the upper half of the data bus (see Figure 3-2). BHE low enables the upper bank, while A0 high disables the lower bank. The data value from the lower bank is ignored during a bus read cycle. A0 high prevents a write operation from destroying data in the lower bank.

To access even-addressed16-bit words (two consecutive bytes with the least-significant byte at an even address), information is transferred over both halves of the data bus (see Figure 3-3). A19:1 select the appropriate byte within each bank. A0 and BHE drive low to enable both banks simultaneously.

Odd-addressed word accesses require the BIU to split the transfer into two byte operations (see Figure 3-4). The first operation transfers data over the upper half of the bus, while the second op- eration transfers data over the lower half of the bus. The BIU automatically executes the two-byte sequence whenever an odd-addressed word access is performed.

3-2