Control ARM (Raspberry Pi) Timer in assembly using functions

The aim of this tutorial is to explain Baking Pi - operating Systems development, Lesson 4 OK04 published by Alex Chadwick, University of Cambridge.

1. First function, load Timer address in r0

1    .global GetSystemTimerBase // this function will set the Timer address in ro

2    GetSystemTimerBase:

3    ldr r0, =0x20003000
4    mov pc, lr //finich the function and go to the next instruction

2. Second Function, GetTimeStamp (the value in counter)

5    .global GetTimeStamp // this function will set the 
6    GetTimeStamp:
7    push {lr} //we store the value of lr because another function will be called inside the current function
8    bl GetSystemTimeBase // this function will set the

STOP /!\ At this point, the current registers values are the following

r0=Time address=0x20003000

9    lord r0, r1, [r0, #4] // load the value in r0+4 (8 bytes) to  r0, r1. r0 will take the lower 4 bytes and r1 the higher 4 bytes

STOP /!\ At this point, the current registers values are the following

r0=lower 4 bytes of the counter 
r1=higher 4 bytes of the counter

10   pop {pc} // return (we have already pushed lr in the stack in line 7)

Let's suppose that the wait time does't exceed 4 bytes (2^32 ms=4294967 and 296 ms), we will so only use the lowest counter's 4 bytes to compare.

2. Coding the wait method

1    .global Wait

2    Wait:
3    delay .req r2 //delay represents the value set in r2
4    mov delay, r0 //r2=r0
5    push {lr} //we save lr because another function will be called
6    bl GetTimeStamp //at this point, r0 contains the lower 4 bytes of the counter (the timestamp)
7    start .req r3 //start represents the value in r3
8    mov start, r0 //r3 (start)=r0 (start timestamp)

9    loop$: //entering a loop
10       bl GetTimeStamp //at this point r0 contains the current lower 4 bytes of the counter (the timestamp)
11       elapsed .req r1 //elapsed represents r1
12       sub elapsed r0,start //r1(elapsed)=r0-r3(start)
13       cmp elapsed, delay //compare r1(elapsed) with r2(delay)
14       .unreq elapsed //Elapsed doesn't represent r1 anymore
15       bls loop$ //stay in the loop if elapsed<delay

16   .unreq delay
15   .unreq start
15   pop {pc}