CS270 Colorado State University

=========
LC3
=========





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Addressing Modes

    Addressing mode indicates where the operand is located.
        

    
    1. Register
    
        
    
        

    
    2. Immediate
    
        
    
        

    
    3. Memory
        The effective address is encoded in instruction 
        (direct or PC-relative).

            

                            
        
        Pointer to effective address is encoded in instruction
        (indirect)
        
            

            

        
        Pointer to the effective address is in a register.
        (relative, or base+offset)

            

            


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traps

    Each trap invokes an operating system service.
    
    In current LC3sim, just modeling the effect of the TRAP and not
    what it does in detail.
    
    TRAP    trap vector
    
    Trap vector table
        Like a virtual method table used to implement Java or C++.
        
        Each entry holds the address of the function to service the trap.
        
        PC is set to the value at trap vector location.


    LC3sim currently implements
        x25     halt program
        
        x23     input character from keyboard with prompt and echo, and
                place the character in the 8 LSB of R0
        
        x21     output character in 8 LSB of R0 to the console display


    


============
LC3 assembly
============
Ch. 7


------------------------------------------
Example: Finding the first 1 bit in a Word
    Input: 0x300A is location in memory of Word to be examined
    Output: R1 contains bit position of first 1 bit in Word or -1 if no 1 bit 
    (Similar to example 4 in Chapter 6)
    
    ------
    C code
        Assume variable w is located at memory location x3009 for 
        the C code to "match" the assembly.
        
        #include <stdio.h>
        int main()
        {
            //short w = 0x0010;
            //short w = 0x0000;
            short w = 0x0020;

            // Put value in R2 "register"
            short R2 = w;

            // R1 iterates from MSB position to LSB position
            int R1;
            for (R1=15; R1>=0; R1--) {
                // If MSB is 1, then number is negative
                if (R2 < 0) { break; }
                // shift word in question one big to the left
                R2 = R2 << 1;
            }

            printf("R1 = %d\n", R1);

        }

    -----------------
    LC3 assembly code

            
            
            
            
            
            
            

    

            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
    
             


    -----------------
    LC3 machine code
        

            
            
            
            
            
            0x3000  0x24??          # R2 = Mem(x300A)
            
            
            
            0x3001  0x5260          # R1 <- 0
            
            
            
            0x3002  0x126F          # R1 <- 15

    
            
            
            0x3003  0x08??          # if (R1<0) goto 0x3009

            
            
            
            0x3004  0x16A0          # R3 <- R2
            
            
            
            
            0x3005  0x08??          # if (R2<0) goto 0x3009
            
            
            
            
            0x3006  0x1482          # R2 <- 2*R2

            
            
            
            0x3007  0x127F          # R1 <- R1 - 1
            
            
            
            
            
            
            0x3008  0x0F??          # goto 0x3003
            
    
            0x3009  0xF025          # HALT
            
            
    
            0x300A 0x0020


------------
LC3 assembly 

    Makes writing machine code easier.
        - Enables the use of labels instead of calculating PC offsets.
        
        - Converts decimal values to binary.
            
        
        - Pseudo-ops
            - indicate where data and instructions should be placed in memory
            

------------
Instructions

    LABEL   OPCODE  OPERANDS    COMMMENTS
    

----------
Pseudo Ops

    ARE NOT translated into machine instructions.

.ORIG address   Tells assembler where to locate the program in
                memory (starting address).
    
                
.FILL value     Store value in the next location in memory.
    


.BLKW n         Set aside a block of n words in memory.
    


.STRINGZ string Store the string, one character per word, in
                memory. Add a word of x0000 after the string.
    
    
    
    
    
    
    

                
.END            Marks the end of the source program (not HALT!)


.EXTERNAL       The label so indicated is allocated in another module
                and therefore cannot be resolved until link time.
    
------------------------
mstrout@cs.colostate.edu, 10/6/08