Intel 80C188XL, 80C186XL First Bus Cycle, A191 D158 BHE Low, Second Bus Cycle, 2 8-Bit Data Bus

BUS INTERFACE UNIT

(X + 1)

First Bus Cycle

Y

X

A19:1	D15:8 BHE
	(Low)
		Second Bus Cycle
	Y + 1
	X + 1

D7:0 A0 (High)

(Y)

X

A19:1	D15:8 BHE	D7:0	A0
	(High)		(Low)

A1108-0A

Figure 3-4. 16-Bit Data Bus Odd Word Transfers

3.2.28-Bit Data Bus

The memory address space on an 8-bit data bus is physically implemented as one bank of 1 Mbyte (see Figure 3-1 on page 3-2). Address lines A19:0 select a specific byte within the bank. Unlike transfers with a 16-bit bus, byte and word transfers (to even or odd addresses) all transfer data over the same 8-bit bus.

Byte transfers to even or odd addresses transfer information in one bus cycle. Word transfers to even or odd addresses transfer information in two bus cycles. The BIU automatically converts the word access into two consecutive byte accesses, making the operation transparent to the program- mer.

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