CS270 Recitation 7 - LC-3 Programming Introduction

Goals

To learn how to write a basic LC-3 program with functions, conditionals, and a loop.
To learn how to use the LC-3 assembler and simulator to debug assembly code.

The Assignment

Make a subdirectory called R7 for the recitation; all files should reside in this subdirectory. Copy the file from the link to the R7 directory, a listing of the code with some comments removed is shown below. R7.asm contains:

	   .ORIG x3000
	   BR Main

; A jump table defined as an array of addresses
Functions  .FILL IntAdd       ; address of add routine      (option 0)
	   .FILL IntSub       ; address of subtract routine (option 1)
	   .FILL IntMul       ; address of multiply routine (option 2)

Main       LEA R0,Functions   ; get base of jump table
	   LD  R1,Option      ; get option to use, no error checking
	   ADD R0,R0,R1       ; add index of array
	   LDR R0,R0,0        ; get address of function
	   JSRR R0            ; call selected function
	   HALT

; Parameters and return values for all functions
Option     .BLKW 1            ; which function to call
Param1     .BLKW 1            ; space to specify first parameter
Param2     .BLKW 1            ; space to specify second parameter
Result     .BLKW 1            ; space to store result

; End reserved section: do not change ANYTHING in reserved section!
;------------------------------------------------------------------------
IntAdd                        ; Your code goes here
			      ; Solution has ~4 instructions
	   RET
;-----------------------------------------------------------------------
IntSub                        ; Your code goes here
			      ; Solution has ~6 instructions
	   RET

;-----------------------------------------------------------------------
IntMul                        ; Your code goes here
			      ; Solution has ~9 instructions
	   RET

;-----------------------------------------------------------------------
	  .END

Use the LC-3 assembler to transform your assembly code into object code that can run on the LC-3 simulator:
```
	$ ~cs270/lc3tools/lc3as R7.asm
	
```
Load the LC-3 simulator and the TA will help you step through an invocation of one of the LC-3 subroutines:
```
	$ ~cs270/lc3tools/lc3sim-tk &
	
```
Implement the IntAdd subroutine, using the following algorithm:
- Load the Param1 parameter into a register
- Load the Param2 parameter into a register
- Add the registers storing Param1 and Param2 into another register
- Store the result into the Result memory location and return
Test the IntAdd subroutine in the simulator using Option = 0, Param1 = 0x1234, and Param2 = 0x3456. The answer in Result should be 0x468A. Try a negative value as well.
Implement the IntSub subroutine, which is a clone of IntAdd, however you must negate the second operand before the addition. Use the 2’s complement to do this, as follows:
- Invert the register storing Param2
- Increment Param2 using an immediate add
Test the IntSub subroutine in the simulator using Option = 1, Param1 = 0x8765, and Param2 = 0x3456. The answer in Result should be 0x530F.
Implement the IntMul subroutine, using this terrible algorithm:
- Initialize a register for the result to zero
- Load the Param1 parameter into a register
- If zero, go to the exit code
- Load the Param2 parameter into a register
- If zero, go to the exit code
- In a loop, add the Param1 to the result, and decrement Param2
- Continue in the loop while Param2 is positive
- In the exit code store the Result value and return
Test the IntMul subroutine in the simulator, using Option = 2, Param1 = 0x1234, and Param2 = 0x0003. The answer in Result should be 0x369C. Try a negative value as well.
Submit to the drop box in RamCT for Recitation 7 and show your code to the TA.