Baking_Pi_DN_OK02

.section .init          /*Creates a section called init */
 .globl _start          /*Ensures the code within this section runs first*/
 _start:                /*Between this section and the next section label*/

 /*we shall assume that the format for ops is z,y,x in that order. For example, add s0,r0,r1 would be add z,y,x, effectively adding x and y to get z. */

ldr r0,=0x20200000      /*LOAD REGISTER: a MNEMONIC that stores the hexadecimal number in register 0*/
                        /*The hex number is a memory address. 20200000 base 16 is the GPIO controller's arbitrary location. */

         /*ENABLING OUTPUT TO THE 16TH PIN*/
mov r1,#1               /*THE OK LED is wired to the 16th pin, so we need to enable the 16th pin. * mov (MOVE) is faster than ldr, since mov doesn't use memory for its op. */
                        /*It moves the hex number 1 into the r1 address.  */
lsl r1,#18              /*LOGICAL SHIFT LEFT: shifts binary representation of first argument, i.e. r1 by second argument, i.e. 18 places. */
                        /*Ergo, from 1 base 2 to 1,000,000,000,000,000,000 base 2 which is 1 with 18 zeros. */

str r1,[r0,#4]          /*STORE REGISTER: adds 4 to the value of register 0, in which we placed the GPIO controller's address, and stores it in register 1. */

/*The 16th pin, the GPIO pin, is now ready, willing, and able. Now we can send the message to turn on. */
/*The funny thing is, we turn off the pin, to turn ON the pin! Silly OS designers! */


//+++++++++++++++++++++++++++++**********************************************++++++++++++++++++++++++++++++
//TURN THE PIN OFF/ LED ON:

mov r1,#1               /*Places a "1" into r1*/
lsl r1,#16              /*Shifts binary representation of 1 and does a logical shift left by 16 places. */

//****** * * * * * *********//
loop$:  // we don't need to keep re-enabling the pin, we just need to send a signal to it, so we only loop the signal sending, not the pin enabling....

str r1,[r0,#40]         /*The "magic" number, this is the address at GPIO Controller + 40 base 10, effectively writing to the GPIO pin.*/
                        //The opposite "magic" number to turn the pin ON, and the LED effectively off.

        /*To wait, after turning the pin off/ LED on in Lesson ok01, one then wastes some time doing nothing, and then turns the pin on/ LED off, and repeat. */

//WAITING


mov r2,#0x3F0000        /*moves 3F0000 base 16 to register 2. This is a simple arbitrary number, decimal 4128768.
wait1$:
sub r2,#1               //SUBTRACT: this subtracts a '1' from the huge number in r2
cmp r2,#0               //COMPARE: this compares the 1st argument with the 2nd, and stores the result in the specialized ergister => CURRENT PROCESS STATUS REGISTER
                        //It remembers which was bigger, smaller or if they were equal.
bne wait1$              //BRANCH IF NOT EQUAL: If the result of the previous comparison was not equal, a simple branch will be performed.


//+++++++++++++++++++++++++++++**********************************************++++++++++++++++++++++++++++++
//TURN THE PIN ON/ LED OFF:
mov r1,#1               /*Places a "1" into r1*/
lsl r1,#16              /*Shifts binary representation of 1 and does a logical shift left by 16 places. */
str r1,[r0,#28]         /*The "magic" number, this is the address at GPIO Controller + 40 base 10, effectively writing to the GPIO pin.*/
                        //The opposite "magic" number to turn the pin ON, and the LED effectively off.
//WAITING ...AGAIN
mov r2,#0x3F0000
wait2$:
sub r2,#1
cmp r2,#0
bne wait2$

b loop$

/*
*OLD CODE from ok01
*loop$:                  /*scoobydoo: labels the next line as scoobydoo. This means the next line is labelled as loop.
* b loop$                /*BRANCH: causes the current line, labelled as "loop". And again, and again.
*/

/*GNU toolchain expects a new line at the EOF (end of file). Failure to do so results in a whiny assembler.*/