8088
Table 1. Pin Description
The following pin function descriptions are for 8088 systems in either minimum or maximum mode. The ‘‘local
bus’’ in these descriptions is the direct multiplexed bus interface connection to the 8088 (without regard to
additional bus buffers).
Symbol
AD7–AD0
Pin No. Type
Name and Function
9–16
I/O ADDRESS DATA BUS: These lines constitute the time multiplexed
memory/IO address (T1) and data (T2, T3, Tw, T4) bus. These lines are
active HIGH and float to 3-state OFF during interrupt acknowledge and
local bus ‘‘hold acknowledge’’.
A15–A8
2–8, 39
O
ADDRESS BUS: These lines provide address bits 8 through 15 for the
entire bus cycle (T1–T4). These lines do not have to be latched by ALE
to remain valid. A15–A8 are active HIGH and float to 3-state OFF
during interrupt acknowledge and local bus ‘‘hold acknowledge’’.
A19/S6, A18/S5, 35–38
A17/S4, A16/S3
O
ADDRESS/STATUS: During T1, these are the four most significant
address lines for memory operations. During I/O operations, these lines
are LOW. During memory and I/O operations, status information is
available on these lines during T2, T3, Tw, and T4. S6 is always low.
The status of the interrupt enable flag bit (S5) is updated at the
beginning of each clock cycle. S4 and S3 are encoded as shown.
This information indicates which segment register is presently being
used for data accessing.
These lines float to 3-state OFF during local bus ‘‘hold acknowledge’’.
S4
S3
Characteristics
0 (LOW)
0
1 (HIGH)
1
S6 is 0 (LOW)
0
1
0
1
Alternate Data
Stack
Code or None
Data
RD
32
O
READ: Read strobe indicates that the processor is performing a
memory or I/O read cycle, depending on the state of the IO/M pin or
S2. This signal is used to read devices which reside on the 8088 local
bus. RD is active LOW during T2, T3 and Tw of any read cycle, and is
guaranteed to remain HIGH in T2 until the 8088 local bus has floated.
This signal floats to 3-state OFF in ‘‘hold acknowledge’’.
READY
INTR
22
18
I
I
READY: is the acknowledgement from the addressed memory or I/O
device that it will complete the data transfer. The RDY signal from
memory or I/O is synchronized by the 8284 clock generator to form
READY. This signal is active HIGH. The 8088 READY input is not
synchronized. Correct operation is not guaranteed if the set up and hold
times are not met.
INTERRUPT REQUEST: is a level triggered input which is sampled
during the last clock cycle of each instruction to determine if the
processor should enter into an interrupt acknowledge operation. A
subroutine is vectored to via an interrupt vector lookup table located in
system memory. It can be internally masked by software resetting the
interrupt enable bit. INTR is internally synchronized. This signal is active
HIGH.
TEST
23
I
TEST: input is examined by the ‘‘wait for test’’ instruction. If the TEST
input is LOW, execution continues, otherwise the processor waits in an
‘‘idle’’ state. This input is synchronized internally during each clock
cycle on the leading edge of CLK.
2
INTEL [ INTEL ]
