Difference between revisions of "CSC103: DT's Notes 1"

From dftwiki3
Jump to: navigation, search
Line 1,233: Line 1,233:
  
 
We won't learn all of the them.  The ones we touched on in the lab is sufficient to understand how a processor works.
 
We won't learn all of the them.  The ones we touched on in the lab is sufficient to understand how a processor works.
 +
 +
====Studying an Assembly Language Program====
 +
 +
Let's take the solution program for the last problem of the [[CSC103 Assembly Language Lab (version 2) 2013| laboratory]]
 +
you just did and go through it, one instruction at a time.
 +
 +
The program is the following:
 +
 +
<br />
 +
<code><pre>
 +
; Solution for Problem 5
 +
; D. Thiebaut
 +
; First initialize both variables with 55 
 +
start: lod-c 55
 +
        sto var1
 +
        sto var2 
 +
 +
; now loop and increment Location 15 and decrement Location 16
 +
; the loop is endless.
 +
loop: lod 15
 +
        add-c 1
 +
        sto 15
 +
        lod 16
 +
        dec
 +
        sto 16
 +
        jmp loop
 +
        hlt 
 +
@15
 +
0
 +
0
 +
</pre><code>
 +
First, the lines starting with semi colons are comments, and not part of the actual program.  They allow the programmers
 +
to add extra information for other programmers who may read the code.
 +
 +
{|
 +
! instruction
 +
! comment
 +
|-
 +
|
 +
start: lod-c 55
 +
| start: is a label.  It is a word we choose that cannot be confused with an instruction.  It is a way
 +
of documenting the program.  The instruction loads the constant 55 into the AC register.
 +
|-
 +
|
 +
        sto 15
 +
        sto 16 
 +
|
 +
store the contents of the AC register (which now contains 55) at Memory Locations 15 and 16.
 +
|-
 +
|
 +
; now loop and increment Location 15 and decrement Location 16
 +
; the loop is endless.
 +
|
 +
Two comments to indicate what is going on next.
 +
|-
 +
|
 +
loop: lod 15
 +
|
 +
We give a name to the current memory location and call it "loop".  The instruction tells the
 +
processor to go to Address 15 and load the number there into AC.
 +
|-
 +
|
 +
        add-c 1
 +
|
 +
Add the constant 1 to AC.
 +
|-
 +
|
 +
        sto 15
 +
|
 +
Store AC back at memory location 15
 +
|-
 +
|
 +
        lod 16
 +
|
 +
Now load the contents of Memory Location 16 into AC.
 +
|-
 +
|
 +
        dec
 +
|
 +
Decrement AC by 1 (always by 1 for INC and DEC)
 +
|-
 +
|
 +
        sto 16
 +
|
 +
Store AC back at Memory Location 16.
 +
|-
 +
|
 +
        jmp loop
 +
|
 +
Force the processor to set its PC register to the address corresponding to the '''loop''' label.
 +
The processor will go back to this address and execute the '''lod 15''' instruction.
 +
|-
 +
|
 +
        hlt 
 +
|
 +
Halt.  The processor will stop when it reaches this instruction.  But... because we have
 +
a program with an endless or ''infinite'' loop, the processor will never reach this instruction.
 +
Nonetheless, it is good practice to always have a HLT instruction to mark the end of a program.
 +
|-
 +
|
 +
@15
 +
|
 +
We now indicate that we are going to store some variables starting at Address 15.
 +
|-
 +
|
 +
0
 +
|
 +
The word at Memory Location 15 is initialized with 0.
 +
|-
 +
|
 +
0
 +
|
 +
The word at Memory Location 16 (next logical address after 15) is initialized with 0 as well.
 +
|}
 +
  
 
Here we summarize the important points you should remember from this section.
 
Here we summarize the important points you should remember from this section.

Revision as of 11:37, 22 September 2013

--© D. Thiebaut 08:10, 30 January 2012 (EST)



This section is only visible to computers located at Smith College













.

Retrieved from "http://dominiquethiebaut.com/dftwiki3/index.php?title=CSC103:_DT%27s_Notes_1&oldid=17803"