Hardware Reference
In-Depth Information
The heart of every computer is the set of instructions it can carry out. To really
understand a computer, it is necessary to have a good understanding of its instruc-
tion set. In the following sections, we will discuss the most important of the
8088's instructions. Some of them are shown in Fig. C-4, where they are divided
into 10 groups.
The first group of instructions is the copy and move instructions. By far, the
most common is the instruction
MOV
, which has an explicit source and an explicit
destination. If the source is a register, the destination can be an effective address.
In this table a register operand is indicated by an
r
and an effective address by an
e
,
so this operand combination is denoted by
e
r
. This is the first entry in the
Operands
column for
MOV
. Since, in the instruction syntax, the destination is the
first operand and the source is the second operand, the arrow
←
←
is used to indicate
the operands. Thus,
e
r
means that a register is copied to an effective address.
For the
MOV
instruction, the source can also be an effective address and the
destination a register, which will be denoted by
r
←
e
, the second entry in the
Operands
column of the instruction. The third possibility is immediate data as
source, and effective address as destination, which yields
e
←
#
. Immediate data in
the table is indicated by the sharp sign (#). Since both the word move
MOV
and the
byte move
MOVB
exist, the instruction mnemonic ends with a
B
between parenthe-
ses. Thus, the line really represents six different instructions.
None of the flags in the condition code register are affected by a move in-
struction, so the last four columns have the entry ''-''. Note that the move instruct-
ions do not move data. They make copies, meaning that the source is not modified
as would happen with a true move.
The second instruction in the table is
XCHG
, which exchanges the contents of a
register with the contents of an effective address. For the exchange the table uses
the symbol
←
. In this case, there exists a byte version as well as a word version.
Thus, the instruction is denoted by
XCHG
and the
Operand
field contains
r
←
e
.
The next instruction is
LEA
, which stands for Load Effective Address. It computes
the numerical value of the effective address and stores it in a register.
Next is
PUSH
, which pushes its operand onto the stack. The explicit operand
can either be a constant (# in the
Operands
column) or an effective address (
e
in
the
Operands
column). There is also an implicit operand,
SP
, which is not men-
tioned in the instruction syntax. What the instruction does is decrement
SP
by 2,
then store the operand at the location now pointed to by
SP
.
Then comes
POP
, which removes an operand from the stack to an effective
address. The next two instructions,
PUSHF
and
POPF
, also have implied operands,
and push and pop the flags register, respectively.
→
This is also the case for
XLAT
