CS270 Colorado State University









=============
TRAPS
=============

TRAPS are useful because ...
    programmers can use TRAPS instead of accessing device registers directly
    
    TRAPS can be set up to run in privileged mode


privileged mode
    programs have to have certain privileges to access hardware registers
        I/O including memory mapped I/O
        resetting the clock
        HALT
    
    supervisor versus user program
        OS is considered the supervisor program and in charge of TRAPS
    
    
------------------------------        
TRAPS:  1111 0000 0010 0101
        opcode      trap vector
        
    stores linkage in R7
        
        
    
    
    
    loads address of routine, which is stored in trap vector, into PC
        
        


        x0020   |   x0400   |
        x0021   |   x0430   |
        ...
        
        x0025   |   xFD70   |  
    
        
    
    How many traps are possible in LC3?
                
        
    

------------------------------        
Example: Character output (code from book page 227)

            .ORIG   x0430
            ST      R1, SaveR1
TryWrite    LDI     R1, DSR
            BRzp    TryWrite
WriteIt     STI     R0, DDR

Return      LD      R1, SaveR1
            RET

DSR         .FILL   xFE04
DDR         .FILL   xFE06
SaveR1      .BLKW   1
            .END


------------------------------        
Saving and restoring registers

    Character output example saves R1 before using it
        
    
    caller-save versus callee-save
        
        
        x86 convention
        http://pdos.csail.mit.edu/6.828/2004/lec/l2.html
        %eax, %ecx, %edx are "caller-save" registers 
        %ebp, %ebx, %esi, %edi are "callee-save" registers
        
        
        
        
        

------------------------------        
Subroutines
    functions, procedures, methods
        
        
        
        
        
        
    
    Useful
        enables code reuse and as such reduces code duplication
            
            
            
        
        enables decomposition of task into smaller problems
                    
        
        enables interfaces and information hiding
            
            

    Implementation needs
        passing parameters and return values 
            
        
        ability to call routine and return from it
            

------------------------------        
JSR and JSRR

    JSR LABEL       0100 1 PCoffset11
    
        


    JSRR BaseR      0100 0 BaseR 0000000

        

    
    Why is JSRR necessary?
    
    
------------------------------        
LC3 example with subroutines

           



    ; Subroutine multi
    ; (Copied from course slides that come with book)
    ; Multiply 2 positive numbers
    ; Parameters:
    ; In: R1, R2;   Out: R3
    ;
    multi	AND	R3, R3, #0
	        ADD	R4, R1, #0
	        BRz	zero
    loop	ADD 	R3, R2, R3
	        ADD	R1, R1, # -1
	        BRp	loop
    zero	RET
	        .END


    

    Notes about example
        passing parameters through registers
        
        sending return value through register
        
        multi does not call any other routines or use a TRAP so do not have to
        save R7
        
------------------------------        
Example: Factorial


           

    ; Subroutine fact
    ; Compute factorial for parameter.
    ; Parameters:
    ; In: R4;   Out: R5
    ; if (R4==1) R5 = 1
    ; else R5 = R4 * fact(R4-1)
    fact	ST R7, Save7
            ADD R6, R4, #-1
            BRz BASE        ; if R4==1 then goto base case
            
    RECURSE ST R4, SaveR4
            ADD R4, R6, #0  ; R$ = R4-1
            JSR fact
            LD R4, SaveR4
            ADD R1, R4, #0  ; R1 <- R4
            ADD R2, R5, #0  ; R2 <- R5, output of fact call
            JSR multi
            ADD R5, R3, #0  ; R5 <- R1*R2
            JMP DONE
    
    BASE    AND R5, R5, #0
            ADD R5, R5, #1

    DONE	LD R7, Save7
            RET

    SaveR7  .BLKW   1
    SaveR4  .BLKW   1

	        .END

    

    Notes about example
        
    
                
            

        




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mstrout@cs.colostate.edu, 10/27/08