Each bit of the destination operand is anded with the corresponding bit of the source operand. The result is placed in the destination and the appropriate flags are set.
Two other instructions of importance are:
(i25) [<label>] OR <destination>,<source>
(i26) [<label>] XOR <destination>,<source>
The truth tables for each of these instructions is as follows:
AND OR XOR b1 b2 result b1 b2 result b1 b2 result 0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 0 1 1 1 0 0 1 0 1 1 0 1 1 1 1 1 1 1 1 1 0
The AND, OR, and XOR instructions clear the OF and CF flags and modify the SF, ZF, and PF flags based on the value of the destination operand.
P#1: The AX-register contains an arbitrary value. Write an assembly language program segment which will complement the last four bits.
(i27) [<label>] NOT <destination>
Not performs the 1's complement of the operand. No flags are affected in the flags register.
(i28) [<label>] TEST <destination>,<source>
TEST performs the AND operation without changing the destination value.
In this chapter, we are concerned about two types of control
structures:
(1) the decision structure
(2) the loop structure
In order to implement either of these control structures, we need to have some kind of branching capability. The JUMP instructions provide the means of branching.
There are two kinds of jumps:
(1) conditional JUMP - this jump might cause the CS:IP register
pair to be modified. The determination of the jump is dependent on
the current state of the flags register.
(2) unconditional JUMP - always causes the CS:IP register pair to
be modified.
Conditional jumps are divided into three categories:
(1) unsigned integer jumps
(2) signed integer jumps
(3) flag register jumps (i.e. jumps based on a specific flag in
the flags register)
The general form of a conditional JUMP is:
(i29-46) [<label>] <op-code> <short-label>
where op-code is an op-code from one of the following tables:
Mnemonic Interpretation JUMPs If JA Jump if above CF=0 and ZF=0 JNBE Jump if not below nor equal JAE Jump if above or equal CF=0 JNB Jump if not below JB Jump if below CF=1 JNAE Jump if not above nor equal JBE Jump if below or equal CF=1 or ZF=1 JNA Jump if not above JE Jump if equal to zero ZF=1 JZ (same as above) JNE Jump of not equal to zero ZF=0 JNZ (same as above)
Mnemonic Interpretation JUMPs If JG Jump if greater than ZF=0 and SF=OF JNLE Jump if not less nor equal JGE Jump if greater than or equal SF=OF JNL Jump if not less than JL Jump if less than SF<>OF JNGE Jump if not greater nor equal JLE Jump if less than or equal ZF=1 or SF<>OF JNG Jump if not greater than JE Jump if equal to zero ZF=1 JZ (same as above) JNE Jump of not equal to zero ZF=0 JNZ (same as above)
Mnemonic Interpretation JUMPs If JC Jump if carry CF=1 JNC Jump if no carry CF=0 JO Jump if overflow OF=1 JNO Jump if no overflow OF=0 JS Jump if sign negative SF=1 JNS Jump if sign nonnegative SF=0 JZ Jump if zero ZF=1 JNZ Jump if not zero ZF=0 JP/JPE Jump if parity even PF=1 JNP/JPO Jump if parity odd PF=0
Now we can do some serious damage!!!
I would like to proceed in the following manner:
(1) Let's look at some examples using the single-alternative IF
(2) Let's examine how the microprocessor actually transfers
control if the branch is taken
if(condition) then true statement(s) endif
The question that we need to answer is how do we test for the condition. A condition in general has the form: operand1 <relational-op> operand2 where the relational operator can be the standard six relational operators. As an example, we might have the condition: x < y.
What you want to do is start with the expression (e.g. x < y) and then subtract the second expression from the first. This is just algebraic manipulation. So now we have x - y < 0. The flags register will reflect this relationship and you can use the appropriate jump instructions to test and branch accordingly. That is, are you working with signed or unsigned numbers. What are you testing?
Q#1: Let's assume that x and y are unsigned values and we are testing whether x is less than y. We could write x-y < 0 as our condition. If x is 0 and y is 1, what is the value of the CF flag? What instruction would you use to branch to some label if x is less than y? Note: The following code would simulate the above action:
mov ax,x sub ax,y jb xlessy
We notice with the sub instruction that the destination operand is modified. It would be nice to have an instruction that performs the same operation as the subtract instruction without modifying the destination operand especially since compares happen so often. That is why there is the compare instruction with the following form:
(i47) [<label>] CMP <destination>,<source>
This instruction causes the source operand to be subtracted from the destination operand and the flags register reflects this relationship. Neither the source or destination operands are modified.
P#1: Given the following data segment, write an assembly language program segment that will print the smaller of the two numbers X and Y. Get the values from the keyboard.
.data x WORD ? y WORD ?