Cracker Handbook 1.0 part 44 pot

Purpose: To load the register of the extra segment

Syntax:

LES destiny, source

The source operator must be a double word operator in memory. The content
of the word with the larger address is interpreted as the segment address
and it is placed in ES. The word with the smaller address is the
displacement address and it is placed in the specified register on the
destiny parameter.

4.3 Stack instructions

These instructions allow the use of the stack to store or retrieve data.

POP
POPF
PUSH
PUSHF


POP INSTRUCTION

Purpose: It recovers a piece of information from the stack

Syntax:

POP destiny

This instruction transfers the last value stored on the stack to the

destiny operator, it then increases by 2 the SP register. This increase is
due to the fact that the stack grows from the highest
memory segment address to the lowest, and the stack only works with words,
2 bytes, so then by increasing by two the SP register, in reality two are
being subtracted from the real size of the stack.

POPF INSTRUCTION

Purpose: It extracts the flags stored on the stack

Syntax:

POPF

This command transfers bits of the word stored on the higher part of the
stack to the flag register.

The way of transference is as follows:

BIT FLAG

0 CF
2 PF
4 AF
6 ZF
7 SF
8 TF
9 IF
10 DF
11 OF

These localities are the same for the PUSHF command.

Once the transference is done the SP register is increased by 2,
diminishing the size of the stack.

PUSH INSTRUCTION

Purpose: It places a word on the stack.

Syntax:

PUSH source

The PUSH instruction decreases by two the value of SP and then transfers
the content of the source operator to the new resulting address on the
recently modified register.

The decrease on the address is due to the fact that when adding values to
the stack, this one grows from the greater to the smaller segment address,
therefore by subtracting 2 from the SP register what we do is to increase
the size of the stack by two bytes, which is the only quantity of
information the stack can handle on each input and output of information.

PUSHF INSTRUCTION

Purpose: It places the value of the flags on the stack.

Syntax:

PUSHF

This command decreases by 2 the value of the SP register and then the
content of the flag register is transferred to the stack, on the address
indicated by SP.

The flags are left stored in memory on the same bits indicated on the POPF
command.


4.4 Logic instructions

They are used to perform logic operations on the operators.

AND
NEG
NOT
OR
TEST
XOR


AND INSTRUCTION

Purpose: It performs the conjunction of the operators bit by bit.

Syntax:

AND destiny, source

With this instruction the "y" logic operation for both operators is carried
out:

Source Destiny | Destiny

1 1 | 1
1 0 | 0
0 1 | 0
0 0 | 0

The result of this operation is stored on the destiny operator.

NEG INSTRUCTION

Purpose: It generates the complement to 2.

Syntax:

NEG destiny

This instruction generates the complement to 2 of the destiny operator and
stores it on the same operator.

For example, if AX stores the value of 1234H, then:

NEG AX

This would leave the EDCCH value stored on the AX register.

NOT INSTRUCTION

Purpose: It carries out the negation of the destiny operator bit by bit.

Syntax:

NOT destiny

The result is stored on the same destiny operator.

OR INSTRUCTION

Purpose: Logic inclusive OR

Syntax:

OR destiny, source

The OR instruction carries out, bit by bit, the logic inclusive disjunction
of the two operators:

Source Destiny | Destiny

1 1 | 1
1 0 | 1
0 1 | 1
0 0 | 0


TEST INSTRUCTION

Purpose: It logically compares the operators

Syntax:

TEST destiny, source