1LC-3 TRAP RoutinesTextbook Chapter 9Summer 2008CMPE12 – Summer 2008 – Slides by ADB 2System Calls Certain operations require specialized knowledgeand protection: specific knowledge of I/O device registersand the sequence of operations needed to use them I/O resources shared among multiple users/programs;a mistake could affect lots of other users! Not every programmer knows (or wants to know)this level of detail Provide service routines or system calls(part of operating system) to safely and convenientlyperform low-level, privileged operations2CMPE12 – Summer 2008 – Slides by ADB 3System Call 1. User program invokes system call 2. Operating system code performs operation 3. Returns control to user program In LC-3, this is done through the TRAP mechanismCMPE12 – Summer 2008 – Slides by ADB 4LC-3 TRAP Mechanism 1. A set of service routines A part of operating system -- routines start at arbitrary addresses  Convention is that system code is below x3000 Up to 256 routines 2. Table of starting addresses Stored at x0000 through x00FF in memory Called System Control Block in some architectures 3. TRAP instruction Used by program to transfer control to operating system 8-bit trap vector names one of the 256 service routines 4. A link back to the user program Want execution to resume immediately after the TRAPinstruction3CMPE12 – Summer 2008 – Slides by ADB 5Memory map1. User program executes TRAP: load indirect address of TRAP routine code2. Execute TRAP routine and RETurn to instruction following the TRAP in user programCMPE12 – Summer 2008 – Slides by ADB 6TRAPTrap vectoridentifies which system call to invoke8-bit index into table of service routine addresses in LC-3, this table is stored in memory at 0x0000 –0x00FF 8-bit trap vector is zero-extended into 16-bit memory addressWhere to golookup starting address from table; place in PCHow to get backsave address of next instruction (current PC) in R74CMPE12 – Summer 2008 – Slides by ADB 7TRAPCMPE12 – Summer 2008 – Slides by ADB 8Return: RET (JMP R7) How do we transfer control back toinstruction following the TRAP? We saved old PC in R7 JMP R7 gets us back to the user program at the right spot LC-3 assembly language lets us use RET(return) in place of JMP R7 Must make sure that service routine does not change R7, or we won’t know where to return.5CMPE12 – Summer 2008 – Slides by ADB 9Example of TRAP InstructionWhat does this program do?.ORIG x3000LD R2, TERM ; LD R3, ASCII ; AGAIN TRAP x23 ; Input characterADD R1, R2, R0 ; BRz EXIT ; ADD R0, R0, R3 ; Change to lowercaseTRAP x21 ; Output to monitor...BRnzp AGAIN ; TERM .FILL xFFC9 ; -‘7’ASCII .FILL x0020 ; lowercase bitEXIT TRAP x25 ; halt.ENDCMPE12 – Summer 2008 – Slides by ADB 10Output Service Routinestored in table,location x21.ORIG x0430 ; syscall addressST R7, SaveR7 ; save R7 & R1ST R1, SaveR1; ----- Write characterTryWrite LDI R1, CRTSR ; get statusBRzp TryWrite ; look for bit 15 onWriteIt STI R0, CRTDR ; write char; ----- Return from TRAPReturn LD R1, SaveR1 ; restore R1 & R7LD R7, SaveR7RET ; back to userCRTSR .FILL xFE04CRTDR .FILL xFE06SaveR1 .FILL 0SaveR7 .FILL 0.END6CMPE12 – Summer 2008 – Slides by ADB 11TRAP Mechanism Operation1. Lookup starting address.2. Transfer to service routine.3. Return (JMP R7).CMPE12 – Summer 2008 – Slides by ADB 12TRAP Routines and their Assembler Nameswrite a string to the consolePUTSx22print prompt to console,read and echo character from keyboardINx23read a single character (no echo)GETCx20x25x21vectorhalt the programHALToutput a character to the monitorOUTroutinesymbol7CMPE12 – Summer 2008 – Slides by ADB 13Saving and Restoring Registers Must save the value of a register if: Its value will be destroyed by the service routine, and The value will be used after that action Who saves? Caller- vs callee-saved The caller of the service routine? Knows what it needs later, but may not know what gets altered by called routine The called service routine (callee)? Knows what it alters, but does not know what will be needed later by calling routineCMPE12 – Summer 2008 – Slides by ADB 14ExampleWhat’s wrong with this routine?LEA R3, BinryLD R6, ASCII ; char->digit templateLD R7, COUNT ; initialize to 10AGAIN TRAP x23 ; Get charADD R0, R0, R6 ; convert to numberSTR R0, R3, #0 ; store numberADD R3, R3, #1 ; incr pointerADD R7, R7, -1 ; decr counterBRp AGAIN ; more?BRnzp NEXTASCII .FILL xFFD0COUNT .FILL #10Binry .BLKW #108CMPE12 – Summer 2008 – Slides by ADB 15Saving and Restoring Registers Called routine -- “callee-save” Before start, save any registers that will be altered(unless altered value is desired by calling program!) Before return, restore those same registers Calling routine -- “caller-save” Save registers destroyed by own instructions orby called routines (if known), if values needed later Save R7 before TRAP Save R0 before TRAP x23 (input character) Or avoid using those registers altogether Values are saved by storing them in memoryCMPE12 – Summer 2008 – Slides by ADB 16Recommended exercises Ex 9.2, 9.4, 9.5, 9.11 Ex 9.17,