Microprocessor - NEC Advanced Personal Computer System Reference Manual

2.2 MICROPROCESSOR
The functional heart of the Processor PCB is the NEC pPD8086 Microprocessor, an
NEC-manufactured device physically and logically interchangeable with the Intel
8086.
The 8086 is a high-performance 16-bit CPU packaged in a single40-pin Dual In-line
Package (DIP) chip. It has a direct addressing capability to 1 MB of memory on a
20-bit address bus, of which bits 0 through 15 are time-multiplexed for the 16-bit
data bus. It is driven at a 4.9152 MHz clock rate and is used in maximum operating
mode (using an external 8288 bus controller).
In general, the 8086 processes a program by repeated cycling through four clock
steps, Tl through T4:
Tl fetches an instruction from memory
T2 reads in any required operand
T3 executes the instruction
T4 writes any required result.
In the 8086, two separate processors perform these steps independently and simul-
taneously: (1) an execution unit that executes instructions, and (2) a bus interface
unit that fetches instructions and queues them up for use by the execution unit.
All registers and data paths within the execution unit are 16 bits wide for fast
operation. A 16-bit arithmetic-logic unit manages the general registers and instruc-
tion operands and maintains status and control flags. The execution unit is a strictly
internal device and has no connection to the outside world. All instructions and
memory access operations are accomplished by the bus interface unit.
The bus interface unit functions as a good secretary by anticipating the needs of the
execution unit and lining up sequential instructions for ready access. These instruc-
tions are stored in an internal queue RAM with a capacity of six bytes. The bus
interface unit is programmed to keep this RAM filled, fetching two bytes at a time
from even addresses and one byte at a time from odd addresses. When the execution
unit requests a memory or
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read or write, the bus interface unit discontinues
instruction fetching and responds to the execution unit request. If the instruction
executed calls for control transfer to another location, the bus interface unit empties
the queue RAM, fetches the instruction from the new location, and feeds it directly
to the execution unit. Then the bus interface unit proceeds to refill the queue RAM
from sequential instructions from the new location.
Processor PCB
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