Flashing an “LED” (Part 2)

Flashing an “LED” (Part 2) A Better Busy Wait Timer

Computer Systems Lecture 8.5

Busy Wait v1

• Last video we applied the “dumb” busy wait timer:

MOV R2, #5000000 // init number of iterations for timer
MOV R2,R3 // MOV iterations into working register R3

timer1:

SUB R3,R3,#1 // subtract 1 from R3
CMP R3, #0 // compare R3 with #0
BNE timer1 // keep looping until R3 reaches zero

Any issues?

• Yes!
• Consider executing this assembly code on real hardware and ask yourself “how long will the delay be?”
• Imagine you executed the code on two different CPUs?
• EG
• RPi2B (900MHz quad -core ARM Cortex -A7 processor)
• RPi4B (1.5GHz quad core ARM Cortex -A72 processor)
• Will the delay be the same on both?
• Lets see!

Busy Wait Flash on RPi4 (left) vs RPi2 (right)

A Better Busy Wait

• Most CPUs have access to a register that maintains a real time count. microsecond intervals (106 per second)
• In ARMlite, a register maintains a 1 second counter
• accessible via the pre-defined.Time label
• Eg LDR R0,.Time loads the current time in seconds (since Jan 1 2000) into R0
• We can utilise this to produce a hardware independent real-time delay

A Better Busy Wait

psuedocode

Set desired delay time in seconds

start_time = current time in seconds

loop:

now = current time in seconds
remaining_time = (now – start_time)

compare remaining_time, delay

loop if remaining_time <= delay

A Better Busy Wait

• Let’s allocate some variables to registers
• R2 – desired delay
• R3 – now (where the current time will be stored)
• R4 – start time
• R5 – elapsed time (R3-R4)

• Let’s allocate some variables to registers
• R2 – desired delay
• R3 – now (where the current time will be stored)
• R4 – start time
• R5 – elapsed time (R3-R4)

Pause the video and have a go!

You might want to edit the code from the last video, and replace

Busy Wait v1 with new improved Busy Wait

A Better Busy Wait

solution

• Provides an absolute timer (no longer dependent on processor speed!)
• However, our approach is still a form of busy wait!
• CPU still occupied for duration of delay
• Essentially this is polling the timer
• Is there a potentially even better approach?
• Well yeah! We could implement an interrupt-based timer
• We will come back to this later in semester!