M80C286 datasheet(3/60 Pages) INTEL

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M80C286

271103 – 2

Figure 3 Status and Control Register Bit Functions

Flags Word Description

The Flags word (Flags) records specific characteris-

tics of the result of logical and arithmetic instructions

(bits 0 2 4 6 7 and 11) and controls the operation

of the M80C286 within a given operating mode (bits

8 and 9) Flags is a 16-bit register The function of

the flag bits is given in Table 1

Instruction Set

The instruction set is divided into seven categories

data transfer arithmetic shiftrotatelogical string

manipulation control transfer high level instruc-

tions and processor control These categories are

summarized in Table 2

An M80C286 instruction can reference zero one or

two operands where an operand resides in a regis-

ter in the instruction itself or in memory Zero-oper-

and instructions (eg NOP and HLT) are usually one

byte long One-operand instructions (eg INC and

DEC) are usually two bytes long but some are en-

coded in only one byte One-operand instructions

may reference a register or memory location Two-

operand instructions permit the following six types of

instruction operations

Memory to Register

Immediate to Register

Memory to Memory

Immediate to Memory

Table 1 Flags Word Bit Functions

Bit

Name

Function

Position

Carry FlagSet on high-order bit

carry or borrow cleared otherwise

Parity FlagSet if low-order 8 bits

of result contain an even number of

1-bits cleared otherwise

Set on carry from or borrow to the

low order four bits of AL cleared

otherwise

Zero FlagSet if result is zero

cleared otherwise

Sign FlagSet equal to high-order

bit of result (0 if positive 1 if negative)

Overflow FlagSet if result is a too-

large positive number or a too-small

negative number (excluding sign-bit)

to fit in destination operand cleared

otherwise

Single Step FlagOnce set a sin-

gle step interrupt occurs after the

next instruction executes TF is

cleared by the single step interrupt

Interrupt-enable FlagWhen set

maskable interrupts will cause the

CPU to transfer control to an inter-

rupt vector specified location

Direction FlagCauses string

instructions to auto decrement

the appropriate index registers

when set Clearing DF causes

auto increment

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