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BUS INTERFACE UNIT
For word transfers, the word address defines the first byte transferred. The second byte transfer
occurs from the word address plus one. Figure 3-5 illustrates a word transfer on an 8-bit bus in-
terface.
Second Bus Cycle
First Bus Cycle
(X + 1)
(X)
A19:0
D7:0
A19:0
D7:0
A1109-0A
Figure 3-5. 8-Bit Data Bus Word Transfers
3.3
MEMORY AND I/O INTERFACES
The CPU can interface with 8- and 16-bit memory and I/O devices. Memory devices exchange
information with the CPU during memory read, memory write and instruction fetch bus cycles.
I/O (peripheral) devices exchange information with the CPU during memory read, memory write,
I/O read, I/O write and interrupt acknowledge bus cycles. Memory-mapped I/O refers to periph-
eral devices that exchange information during memory cycles. Memory-mapped I/O allows the
full power of the instruction set to be used when communicating with peripheral devices.
I/O read and I/O write bus cycles use a separate I/O address space. Only IN and OUT instructions
can access I/O address space, and information must be transferred between the peripheral device
and the AX register. The first 256 bytes (0–255) of I/O space can be accessed directly by the I/O
instructions. The entire 64 Kbyte I/O address space can be accessed only indirectly, through the
DX register. I/O instructions always force address bits A19:16 to zero.
Interrupt acknowledge, or INTA, bus cycles access an I/O device intended to increase interrupt
input capability. Valid address information is not generated as part of the INTA bus cycle, and
data is transferred only over the lower bank (16-bit device).
3-6
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