Song Liu
<p>      


<h1>Different Memory Addressing ( Examples by LOAD )</h1>


<p>

<h2>Immediate Operand</h2>
The simplest way for an instruction to specify an operand is for the
address part of the instruction actually to contain the sperand itself
rather than an address or other information describing where the operand
is. Such an operand is called an immediate operand because it is
automatically fetched from memory at the same time the instruction
itself is fetched. It is immediately available for use.
<pre>
Ex:   LOAD R1, =100	Load the number 100 to register R1.
</pre>

<p>
<p>
<h2>Direct Addressing</h2>
<p>

Direct addressing is a scheme in which the address specifies which
memory word or register contains the operand.
<p>
For example:
<pre>
1) LOAD R1, 100	  Load the content of memory address 100 to register R1.

2) LOAD R1, R2	  Load the content of register R2 to register R1.
</pre>
<p>
<p>
<h2>Indirect Addressing</h2>

Indirect addressing is a scheme in which the address specifies which
memory word or register contains not the operand but the address of the
operand.
<pre> 
For example:

1) LOAD R1, @100	Load the content of memory address stored at memory 
			address 100 to the register R1.

   			R1	M[100]		M[200]

			-	200		10
   LOAD R1,@100		10	200		10
			


2) LOAD R1, @R2		Load the content of the memory address stored at

			register R2 to register R1.
</pre>			


<p>
<h2>Indexed Addressing</h2>
Addresses have two parts: the number of an index register and a
constant. The address of the operand is the sum of the constant and the
contents of the index register. It contains indexed (direct) addressing,
indexed immediate addressing and indexed indirect addressing.
<pre>
For example:

1) LOAD R1, 100(R2)	Load the content of the memory address which is 
			the sum of 100 and the content of register R2 to
			register R1.

   			R1	R2	M[100]	M[110]

			-	10	200	150
   LOAD R1, 100(R2)	150	10	200	150
			


2) LOAD R1, =100(R2)	Load the sum of 100 and the value of register R2
			to register R1.


3) LOAD R1, @100(R2)	Load the content of the memory address stored at
			the memory address which is the sum of 100 and 
			the number in register R2 to the register R1.

   			R1	R2	M[100]	M[110]	M[150]

			-	10	200	150	300
   LOAD R1, @100(R2)	300	10	200	150	300
			
</pre>


<h2>Stack Addressing</h2>
<P>

Computers that are stack-oriented have an instruction to push the
contents of a memory location or a register onto the stack. Instructions
with no addresses are used in conjunction with a stack. This form of
addressing specifies that the two operands are to be popped off the
stack, one after another, the operation performed and the result pushed
back onto the stack.