Intel 80286 User Manual

OVERVIEW OF NUMERIC PROCESSING
As shown in figure 1-2, the 80287 NPX is divided internally into two processing elements; the Bus
Interface Unit (BIU) and the Numeric Execution Unit (NEU). The two units operate independently
of one another: the BIU receives and decodes instructions, requests operand transfers with memory,
and executes processor control instructions, whereas the
NEU processes individual numeric
instructions.
The
BIU handles all of the status and signal lines between the
80287 and the 80286. The NEU executes
all instructions that involve the register stack. These instructions include arithmetic, logical, transcen-
dental, constant, and data transfer instructions. The data path in the
NEU is 84 bits wide (68 fraction
bits, 15 exponent bits, and a sign bit), allowing internal operand transfers to be performed at very high
speeds.
The 80287 executes a single numeric instruction at· a time. Before executing most ESC instructions,
the 80286 tests the BUSY pin and, before initiating the command, waits until the 80287 indicates that
it
is not busy. Once initiated, the 80286 continues program execution, while the 80287 executes the
numeric instruction. Unlike the 8087, which required
aWAIT instruction to test the BUSY signal
before each ESC opcode, these WAIT instructions are permissible, but not necessary,
in 80287 programs.
In all cases, a WAIT or ESC instruction should
be inserted after any 80287 store to memory (except
FSTSW or FSTCW)or load from memory (except FLDENV, FLDCW, or FRSTOR) before the 80286
reads or changes the memory value.
When needed, all data transfers between memory and the 80287
NPX are performed by the 80286
CPU, using its Processor Extension Data Channel. Numeric data transfers performed
by the 80286
use the same timing as any other bus cycle, and all such transfers come under the supervision of the
DATA
STATUS
ADDRESS
OPERANDS
QUEUE
MICROCODE
CONTROL
UNOT
T
•
G
w
o
•
o
II
I- III
1-----;'51
REGISTER STACK
_____ L _____ ...!!II!!. ____ .J
Figure 1-2. 80287 NPX Block Diagram
1-7
G30108
pcjs.org

Intel 80286 Specifications

General

Processor Type	Microprocessor
Clock Speed	6 MHz to 25 MHz
Transistor Count	134, 000
Addressable Memory	16 MB
Instruction Set	x86
Architecture	x86
Data Bus Width	16-bit
Address Bus Width	24-bit
Operating Modes	Real mode, Protected mode
Process Technology	1.5 µm
Voltage	5V
Introduced	1982
Package	68-pin PGA