1. Overview Projects The Assembly Process Programmed I/O Interrupt Driven I/O

2. LC-3 Assembly Language LABEL OPCODE OPERANDS ;COMMENTS optionalmandatory Syntax: Symbol Table: Symbol Address Start x3000 Test x3005 Next x300A Number x3010 String x3011

3. An Assembly Language Program ; ; Program to multiply a number by the constant 6 ;.ORIGx3050 LDR1, SIX LDR2, NUMBER ANDR3, R3, #0; Clear R3. It will ; contain the product. ; The inner loop ; AGAINADDR3, R3, R2 ADDR1, R1, #-1; R1 keeps track of BRpAGAIN; the iteration. ; HALT ; NUMBER.BLKW1 SIX.FILLx0006 ;.END Symbol Table: Symbol Address AGAINx3053 NUMBERx3057 SIXx3058

4. Count the occurrences of a character in a file (1 0f 2). ; ; Program to count occurrences of a character in a file. ; Character to be input from the keyboard. ; Result to be displayed on the monitor. ; Program only works if no more than 9 occurrences are found. ; ; Initialization ;.ORIGx3000 ANDR2, R2, #0; R2 is counter, initially 0 LDR3, PTR; R3 is pointer to character file GETC; R0 gets input character LDRR1, R3, #0; R1 gets first character from file ; ; Test character for end of file ; TESTADDR4, R1, #-4; Test for EOT (ASCII x04) BRzOUTPUT; If done, prepare the output ; ; Test character for match. If a match, increment count. ; NOTR1, R1 ADDR1, R1, R0; If match, R1 = xFFFF NOTR1, R1; If match, R1 = x0000 BRnpGETCHAR; If no match, do not increment ADDR2, R2, #1 ; ; Get next character from file. ; GETCHARADDR3, R3, #1; Point to next character. LDRR1, R3, #0; R1 gets next char to test BRnzpTEST

5. Count the occurrences of a character in a file (2 of 2). ; ; Output the count. ; OUTPUTLDR0, ASCII; Load the ASCII template ADDR0, R0, R2; Covert binary count to ASCII OUT; ASCII code in R0 is displayed. HALT; Halt machine ; ; Storage for pointer and ASCII template ; ASCII.FILLx0030; ASCII offset PTR.FILLx4000; PTR to character file.END

6. One Pass vs Two Pass Assemblers Two Pass – Checks for syntax errors and builds the Symbol Table during first pass, resolves operand addresses during second pass. One Pass – Checks for syntax errors, builds the Symbol Table, and resolves operand addresses during the first pass. So why have a two pass?

7. More than One Object (Load) File Symbol Table Symbols Externals Exports Addresses Start x3000 Number x300A Data ? Value ? The “Linker/Loader” would generate another “global table to resolve Externals & Exports at Load time

8. Input / Output Memory Mapped I/O – A section of the memory address space is reserved for I/O Registers rather than general memory locations. Think of it as “pseudo” memory. The same instructions are used for general programming and I/O programming. Non-Memory Mapped I/O – There is a separate address space for I/O programming, and an entirely separate set of I/O Instructions.

9. LC-3 has Memory Mapped I/O LC-3 Memory Layout: x0000 – x00FF Trap vectors x0100 – x2FFF System Programs & Data x3000 – xFDFF User Programs Area xFE00 – xFFFF I/O Programming “Registers”

10. Synchronous vs Asynchronous I/O Synchronous – data could be expected to be available when the program wanted it – like data in memory. This is not realistic for I/O Asynchronous – computer is generally much faster than I/O so program must wait until data is available or data has been taken. “Handshaking” is used to ensure that data is available or I/O device is ready

11. Polling vs Interrrupt Driven I/O Polling – program checks handshaking signals to find when data is available of device is done (typically a loop in the program) Interrupt – program initiates I/O and waits until data is available (typically goes to sleep until the operating system wakes the program up)

12. Keyboard I/O KBDR: Assigned to xFE02 Data is in KBDR[7:0] Read only Register KBSR: Assigned to xFE00 Status is in KBSR[15] Set to “1” when new data is ready Cleared when data is read

13. Simple Input from Keyboard START LDI R1, A ; Test for BRzp START ; character input LDI R0, B BRnzp NEXT_TASK ; Go to the next task A.FILL xFE00 ; Address of KBSR B.FILL xFE02 ; Address of KBDR

14. Monitor I/O DDR: Assigned to xFE06 Data is in DDR[7:0] DSR: Assigned to xFE04 Status is in DSR[15] Set to “1” when data is picked up Cleared when new data is written

15. Simple Ouput to Monitor START LDI R1, A ; Test to see if BRzp START ; output register is ready STI R0, B BRnzp NEXT_TASK A.FILL xFE04 ; Address of DSR B.FILL xFE06 ; Address of DDR

16. Echo from Keyboard to Monitor START LDI R1, KBSR ; Test for character input BRzp START LDI R0, KBDR ECHO LDI R1, DSR ; Test output register ready BRzp ECHO STI R0, DDR BRnzp NEXT_TASK KBSR.FILL xFE00 ; Address of KBSR KBDR.FILL xFE02 ; Address of KBDR DSR.FILL xFE04 ; Address of DSR DDR.FILL xFE06 ; Address of DDR

17. The I/O Routine for the LC-3 Keyboard START ST R1,SaveR1 ; Save registers needed ST R2,SaveR2 ; by this routine ST R3,SaveR3 ; LD R2,Newline L1 LDI R3,DSR BRzp L1 ; Loop until Monitor is ready STI R2,DDR ; Move cursor to new clean line ; LEA R1,Prompt ; Starting address of prompt string Loop LDR R0,R1,#0 ; Write the input prompt BRz Input ; End of prompt string L2 LDI R3,DSR BRzp L2 ; Loop until Monitor is ready STI R0,DDR ; Write next prompt character ADD R1,R1,#1 ; Increment Prompt pointer BRnzp Loop ; Get next prompt character ; Input LDI R3,KBSR BRzp Input ; Poll until a character is typed LDI R0,KBDR ; Load input character into R0 L3 LDI R3,DSR BRzp L3 ; Loop until Monitor is ready STI R0,DDR ; Echo input character ; L4 LDI R3,DSR BRzp L4 ; Loop until Monitor is ready STI R2,DDR ; Move cursor to new clean line LD R1,SaveR1 ; Restore registers LD R2,SaveR2 ; to original values LD R3,SaveR3 BRnzp NEXT_TASK ; Do the program's next task ;

18. The I/O Routine for the LC-3 Keyboard (2) SaveR1.BKLW 1 ; Memory for registers saved SaveR2.BKLW 1 SaveR3.BKLW 1 DSR.FILL xFE04 DDR.FILL xFE06 KBSR.FILL xFE00 KBDR.FILL xFE02 Newline.FILL x000A ; ASCII code for newline Prompt.STRINGZ "Input a character>"

19. Keyboard I/O KBDR: Assigned to xFE02 Data is in KBDR[7:0] Read only Register KBSR: Assigned to xFE00 Status is in KBSR[15] Set to “1” when new data is ready Cleared when data is read Interrupt Request is in KBSR[14]

20. Monitor I/O DDR: Assigned to xFE06 Data is in DDR[7:0] DSR: Assigned to xFE04 Status is in DSR[15] Set to “1” when data is picked up Cleared when new data is written Interrupt Request is in DRS[14]