index.md (1649B)
1 +++ 2 title = 'Program-controlled I/O' 3 +++ 4 # Program-controlled I/O 5 “a program that performs all functions needed to realise the desired action” 6 7 difference in speed between processor and I/O devices, need to synchronise transfer of data 8 9 solution — signalling protocol (wait for signal from device, a status flag) 10 11 polling — checking the device’s status flag 12 13 ## Example: RISC-style I/O 14 reads from keyboard, echoes back to display. end on newline 15 16 1. Point register R2 to address of first location in main memory where read characters will be stored 17 18 2. Load newline character (terminator) into R3 19 3. Start input loop 20 1. Move keyboard status into R4 21 2. R4 = R4 AND #2 22 - the KIN status flag is the second byte of the value in R4 — so R4 looks like `...0X` or `...1X` 23 - #2 is 10 in binary 24 - ANDing them together gives you the state of the KIN flag (i.e. 0X AND 10 = 0X) 25 26 3. If [R4] is 0 (KIN is false), jump to step 1 of loop 27 4. Load `KBD_DATA` byte into R5 (clearing KIN to 0) 28 5. Store R5 into address in R2 29 6. Increment pointer in R2 (to store next character) 30 7. Start output loop 31 1. Load `DISP_STATUS` byte into R4 32 2. R4 = R4 AND #2 33 34 - the DOUT flag is the third byte of the value in R4 — so R4 is either …0XX or …1XX 35 - #4 is 100 in binary 36 - ANDing them together gives you the state of the DOUT flag (i.e. 0XX AND 100 = 0XX) 37 38 3. If [R4] is 0, jump to step 1 of loop 39 8. Store byte in R5 into `DISP_DATA` 40 9. If the Terminator isn’t in R5 ([R5] ≠ [R3]), jump to step 3 (input loop) 41 42  43 44 