Zenith Z-100 PC series Service Manual page 90

Page
4-4
Detailed Circuit Description
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Table 4-1 (continued): 8088-2 Microprocessor Signals
PIN SIGNAL
NUMBER NAME DESCRIPTION
30 RQ*jGTI*
This signal is used by the 8087 coprocessor
(U234) to gain control of the busses from the
8088-2 (U235).
31
RQ* jGTO* Not connected in the Z-158.
32 RD* Not connected in the Z158.
33 MNjMX* This pin, which is grounded in the Z-158,
informs the microprocessor that it is operat­
ing in a maximum system mode.
34 HIGH This signal is in a high state in the Z-158.
35-38 A16-A19
The high-order addresses or segment ad ­
dresses.
40
VCC +5 VDC power supply.
The Instruction Cycle
The first operation in the instruction cycle is to load the contents of the
internal program counter register into the address outputs (ADO through
AD7). The program counter also keeps track of the address of the next
instruction stored in memory.
When the address is stable a t the outpu t pins, the address is la tched in to
external latches. The output pins are now free to transfer data. The
program counter is incremented so that it indicates the loca tion of the
next instruction.
After the address is latched, a combination of the SO, Sl, and S2 signals is
used to generate the appropriate function, such as "read from memory."
When this signal occurs, the data from the addressed memory location is
received and decoded by the microprocessor. In cases where the
instruction is more than one byte long, the cycle is repeated to load the
rest of the instruction.