calculate value to input for sound delay in a assembly program

Question

I was looking at a program that is about a sound note sequencer that plays different sound notes with different duration, in the middle of some sound notes there will be a delay for like 0.5s or 0.25s, then I cannot figure out a way to calculate the value to load into the register to achieve the duration that I want.

I have searched wiki and some other online resoures such as from this website : https://www.avr-tutorials.com/assembly/calculating-execution-time-sequential-code to find out how to calculate the execution time for a function.

delay:
  push {lr}
  ldr r10, =12000 @ in fact this function will result in a delay of 0.25s
  b loop

loop:
  ..... @ some instructions to play sound
  subs r10, #1 @ in each loop the time counter is subtracted by 1
  cmp r10, #0 @ when time counter reaches zero, branch to a end function
  beq end
  b loop @ loop the function until time counter hits zero

for example, the above function with register r10 as a time counter, and in another loop function,a value of 0 (meaning 0 Hz sound is input into the register to create a delay between sound notes) is added to register r0 by mov r0, #0 , and the I want to find out what value do I need to load into the time counter r10 in order to let me play the 0 Hz sound for like 0.5s or 0.25s and how do I calculate this value to be loaded in. I don't know what is the exact way of saying this but perhaps I will phrase it as " calculating delay time in a program?"

The delay function that I found somewhere else will in fact really result in a 0.25s delay but I don't understand how does it work. I am guessing it takes 0.25s for the time counter r10 to reach 0 by subtracting 1 from it in each loop. But I really cannot figure out how does this calculation comes about.

Thank you.

Answer 1

Just answering on ..in fact really result in a 0.25s delay but I don't understand how does it work part (in fact, this is the only question I can recognise as a question):

A CPU instruction takes some fixed time to execute(*) on a given CPU clocked at a given frequency. Knowing this data you can calculate the number of loop iterations necessary to get the delay you need. It's just a simple arithmetic task.

(*) This does not hold for most modern CPUs and this method of implementing a delay is strongly discouraged for a number of reasons.