Information Technology Reference
In-Depth Information
instructions) is stored.
•
Data segment (DS register) - defines where data from the program will be stored (DS
stands for data segment register).
•
Stack segment (SS register) - defines where the stack is stored.
•
Extra segment (ES).
All addresses are with reference to the segment registers.
The 8086 has a segmented memory, these registers are used to manipulate these seg-
ments. Each segment provides 64 K of memory, this area of memory is known as the current
segment.
Table A.1
Processor flags
Bit Flag
position
Name
Description
C
0
Set on carry
Contains the carry from the most significant bit (left
hand bit) following a shift, rotate or arithmetic op-
eration.
A
4
Set on 1/2 carry
S
7
Set on negative result
Contains the sign of an arithmetic operation (0 for
positive, 1 for negative).
Z
6
Set on zero
result
Contains results of last arithmetic or compare result
(0 for nonzero, 1 for zero).
O
11
Set on overflow
Indicates that an overflow has occurred in the most
significant bit from an arithmetic operation.
P
2
Set on even parity
D
10
Direction
I
9
Interrupt enable
Indicates whether the interrupt has been disabled.
T
8
Trap
Memory Addressing
There are several methods of accessing memory locations, these are:
•
Implied addressing which uses an instruction in which it is known the which registers are
used.
•
Immediate (or literal) addressing uses a simple constant number to define the address
location.
•
Register addressing which uses the address registers for the addressing (such as AX, BX ,
and so on).
•
Memory addressing which is used to read or write to a specified memory location.
