Intel 210200-002 manual Flag Register File

ARCHITECTURE AND INSTRUCTIONS

Programs can be dynamically relocated by changing the segment registers, provided the program itself does not load or manipulate the segment registers;

Flag Register File

Six flags provide processor status informa- tion (Fig. 2-2). Five are the 8080/8085 flags and usually reflect the status of the latest arithmetic or logical operation. The sixth, an OVERFLOW flag, reflects asigned overflow condition.

The 8088 also contains three flags that con- trol processor operations. These are the DIRECTION flag, which controls the direc- tion of the string manipulations; the INTER~ RUPT FLAG, which enables or disables external interrupts; and the TRAP flag, which puts the processor into a single-step mode for program debugging.

A more detailed discussion of the flags ,follows:

1)If AF (the auxiliary carry flag) is set, there has been a carry out of the low nibble (the low order 4-bits of a byte) into the high nib- ble or a borrow from the high nibble into the

REGISTER	OPERATIONS .
AX	Word Multiply, Word Divide,
	Word 1/0
AL	Byte Multiply, Byte Divide, Byte
	1/0, Translate, Decimal
	Arithmetic
AH	Byte Multiply, Byte Divide
BX	Translate
CX	String Operations, Loops
CL	Variable Shift and Rotate
OX	Word Multiply, Word Divide,
	Indirect I/O
SP	Stack Operations
SI	String Operations
01	String Operations

Figure 2-3. Implicit Use of General Registers

low nibble of an 8-bit quantity (low-order byte of a 16-bit quantity). This flag is used by decimal arithmetic instructions.

2)If CF (the carry flag) is set, there has been a carry out of, or a borrow into, the high- order bit of the result (8- or 16-bit). The flag is used by instructions that add and subtract multibyte numbers. Rotate instructions can also isolate a bit in memory or a register by placing it in the carry flag.

3)If OF (the overflow flag) is set, an arith- metic overflow has occurred; that is, a signifi- cant digit has been lost because the size of the computation exceeded the capacity of its des- tination location. An optional Interrupt On Overflow instruction generates an interrupt in this situation.

4)If SF (the sign flag) is set, the high-order bit of the result is a 1. Since negative binary numbers are represented in the 8086 and 8088

in standard two's complement notation, SF indicates the sign of the result (0 =positive, 1 =negative).

5)If PF (the parity flag) is set, the result has even parity, an even number of I-bits. This flag can be used to check for data transmis- sion errors.

6)If ZF (the zero flag) is set, the result of the operation is o.

Three additional control flags (Fig. 2-2) can be set and cleared by programs to alter pro- cessor operations:

1)Setting DF (the direction flag) causes string instructions to auto-decrement, that is, to process strings from high addresses to low addresses, or from "right to left". Clearing DF causes string instructions to auto- increment, or to process strings from '~left to right."

2)Setting IF (the interrupt-enable flag) allows the CPU to recognize external (mask- able) interrupt requests. Clearing IF disables these interrupts. IF has no effect on either nonmaskable external or internally generated interrupts.

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