1. Interrupts & Syscalls

2. Interrupts
Interrupt : (aka Exceptions) Signal to CPU that something needs to happen
Hardware Interrupts
Input/output ready
Ctrl-C
Code needs help
Trap from math overflow
Request for OS

3. ARM Users ARM Modes Triggered by different interrupts
ARMSim does not model other modes

4. Interrupt Handling

5. Interrupt Handling
Interrupts complete before normal processing resumes
May be interrupted by higher priorty interrupts

6. Interrupts ARMv7 maintains shadow registers for interrupts
Avoid save/reload

7. Interrupts ARMv8 On Exception, processor stores
Current instruction to Exception Link Register ELR
Exception readon to Exception Syndrome Register ESR
Then branches to known address for OS to run handler

8. Interrupts
Vectored Interrupts: Each interrupt matched to special memory address
Addresses contain addresses of Interrupt Service Routines (ISR)

9. Interrupts On exception Processor loads address given in vector
Loads that address and sets PC to it
MEMORY
0x0000
0x01A0
0x0004
0x0200
0x0008
0x04F2
0x000C
0x0500 …
Code for Reset
Code for SWI

10. ARM Linux Syscalls

11. Syscalls Operating system has control over hardare resources
Filesystem
Memory
Etc…

12. Syscalls System Calls : Software generated exception
Ask OS to do a job it can’t

13. Syscalls Syscalls defined by OS
Linux different than Windows
Way to call them defined by platform convention

14. ARM Linux Syscalls
SWI 0 used to trigger all system calls:

15. ARM Linux Syscalls
R7 used to specify syscall to do

16. Syscalls
Basic system call cheat sheet:

17. Syscalls File Handles: Number identifying “file”
File 0 is always stdin (console input)
File 1 is stdout (console output)
File 2 is stderr (console output but can be treated different)

18. Hello World
Write to console:

19. Echo Input
Read a line to buffer…

20. Echo Input
…write it out

21. Real Files
Open takes file name
Via address to string in memory

22. Real Files Open returns fileHandle Number identifying opened file
-1 if error
Should be 3 for your first opened file
Store fileHandle for use with read/write

23. Real Files
Flags : Mode to open file in

24. Real Files
Mode : Use means 0 : Octal number 6 : you can read write 4 : group can read 4 : others can read

25. Real Files
File write: