Assembly Language Examples

Posted by Jeff Schuler on Sep 25, 2010 in Uncategorized |

This article gives another step-by-step lesson about assembly language programming.  In order to follow along with this article, you should have this previous article on hand:  An Assembly Language Example

Refer to that program and do the following:

Rewrite step 3 above using 32 and 96 instead of C6 and C7.

This sounds like a good idea at first. Only about half the number of instructions will be needed as we move 2 bytes at a time instead of one. But wait! How do we move an odd number of bytes? We can’t move only the 5th byte using these two instructions. For this exercise it is not fair to revert back to the previous instructions for the 5th byte (although in an actual program that would be a very good idea). Read the code below to find out!

E425: 32 E24E                              load H:X from ni[0-1]
96 0188                                           store H:X into n[0-1]
32 E250
96 018A
32 E251
96 018B

This is expressed more cryptically in a simulator/debugger memory display, but is the same info:

E420: .. .. .. .. .. 32 E2 4E
E428: 96 01 88 32 E2 50 96 01

E430: 8A 32 E2 51 96 01 8B ..

Another Assembly Language Task:

Perform I = 3*J – K + 21, where I, J, and K are variables. We will need some new instructions. They are:

CB hh ll Add value at location hhll into A
C0 hh ll Subtract value at location hhll from A

Step1. Decide where to place I, J, and K in memory. Remember they must be in RAM.

019E: ?? Location for I showing its value is unknown
019F: ?? Location for J
01A0: ?? Location for K

Usually we just place all our variables in consecutive RAM locations.

Step2. Decide where in memory to place the constant 21. It must be in ROM.

E0E0: 15                        Note the value looks different in hex!

Step3. Decide where to place the program and write it.

E10C: C6 019F                    load A with value from J
CB 019F                                add in another J to form 2*J in A
CB 019F                                add in another J for 3*J
C0 01A0                              subtract K from 3*J in A
CB E0E0                              add the constant 21 into A
C7 019E                              store the result into I

Step4. Define the starting address in the reset address in ROM.

FFFE: E10C

You should enter these values in the S08 simulator memory, and run the program a couple of times. REMEMBER to enter test values into the variables J and K before running the program. This section of code assumes that those values get placed there by sections of code not shown. To test this section you must provide some values to test. Verify that the value stored into I by the last instruction is correct for the values of J and K that you provide.

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