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Topic: So How Does an iPod Actually Work? (Read 15957 times) previous topic - next topic
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So How Does an iPod Actually Work?

Reply #25 –
All the code on the 'ROM' does is load the code stored on the firmware partition and run it. I think it also has disk mode and a few other features that are only activated by holding special key combinations.

In the process of installing rockbox, you must read the contents of this partition (on your PC), and modify it with a special tool that replaces it with the rockbox bootloader and appends the apple firmware. You then write this back to the firmware partition.

The actual Rockbox firmware is stored in a file on the FAT32 partition you can see when you look at your iPod in my computer. This allows for easy upgrading etc.

When you restart your iPod after installing rockbox, the software on the ROM executes the rockbox bootloader stored on the firmware partition, which depending on your selection either loads the rockbox firmware from the FAT32 partition (the bootloader includes a simple FAT32 'driver') or executes the original apple firmware, which is still in the firmware partition.

So How Does an iPod Actually Work?

Reply #26 –
Thank you very much, David!

Although I feel rather dense, because of all the explaining you kind folks have had to do, I feel really good to be gaining an understanding of how this thing works.  A lot of my problem has to do with "jargon," because I'm not (yet) that involved with the innards of computers.

In particular, the term "firmware" has caused me some confusion.  I used to think this was code that went onto ROM, and could be updated from time to time.  But it sounds like "firmware" can now also mean software that performs functions that used to be done by code on ROM (or something like that).

Anyway, thanks again to all who have persevered in steering me through this primer on how an iPod actually works!

So How Does an iPod Actually Work?

Reply #27 –
Firmware means different things in different contexts.  Mosts developers would probably think of Rockbox on the Ipod as software, and the USB stuff as firmware.  At least for practical purposes, Rockbox, RetailOS and Linux are just normal operating systems, although very much stripped down and customized for the odd kind of computer they run on. 

On other players, Rockbox really does run from ROM (you "flash" rather then "install" it), so then it might be a firmware.  And on players like the Sansa, where there is no separate ROM and hard disk, its even more confusing, since Rockbox, your music and the "firmware" all run from the same flash chip which mounts to look like a harddisk!  People have bricked those players by formating the ROM 

For this reason I tend to call things that can be accessed on a partition in Windows or Linux as software, and things that are stored in a place that cannot be mounted as firmware, but thats mostly just my preference.

So How Does an iPod Actually Work?

Reply #28 –
Hey, that really helps.

Now I see how the terminology got to be used the way it is, and - especially - how it pertains to the case at hand (the iPod).

My thanks to you and the other contributors to this thread who have made this iPod much more understandable to me.

I'm happy to learn new things, and I'm happy knowing that some people will take the time to help out others, even when their patience might be tested.

So How Does an iPod Actually Work?

Reply #29 –
Quote from: Mike Giacomelli on
The iPod OS is actually stored on the hard disk.  The EEPROM has a basic BIOS like system, some programs that can be used for testing, and the USB code.  Once this loads, it passes control to whatever software is installed on the disk, and is not used again until you reboot.  As you say, its actually like a PC based in a small box.


While most of the memory is used for buffering, the system also runs program code from RAM too, so its not all buffer.

chelgrian:  You an Apple developer?


The OS (firmware) might be stored on the hard drive but it is loaded onto the RAM while running.  I found that the 5G iPod has a 1MB (or 512KB) NOR flash ROM chip that contains the bootloader program that tells the iPod to load the OS from the hard drive.  A small portion of RAM is used to hold the iPod OS once it is loaded.  After a 5G/5.5G iPod is turned off for a period of 24 hours, all information is deleted from the RAM thus causing the iPod to perform a quick boot.  If the iPod is turned off then turned back on in less than a 24 hour period, the OS is already loaded from the RAM thus making boot times instant.

The 5G/5.5G iPods contain two ARM 7TDMI (derived) processors running at 90MHz a piece.

So, from the looks of it, not all of the RAM is used for buffer memory.  I guess some of these features were previously covered, I just wanted to clear some things up as to why the iPod will pretty much turn completely off after being in sleep mode for more than 24 hours or how the iPod actually works.

So How Does an iPod Actually Work?

Reply #30 –
Quote from: kornchild2002 on
The OS (firmware) might be stored on the hard drive but it is loaded onto the RAM while running.  I found that the 5G iPod has a 1MB (or 512KB) NOR flash ROM chip that contains the bootloader program that tells the iPod to load the OS from the hard drive.  A small portion of RAM is used to hold the iPod OS once it is loaded.  After a 5G/5.5G iPod is turned off for a period of 24 hours, all information is deleted from the RAM thus causing the iPod to perform a quick boot.  If the iPod is turned off then turned back on in less than a 24 hour period, the OS is already loaded from the RAM thus making boot times instant.


It doesn't directly reboot.  It actually dumps it's state to the disk and then powers down to save battery.  Presumably they included the real time clock so that they'd have something to trigger the power off interrupt.

So How Does an iPod Actually Work?

Reply #31 –
Many thanks to the Kornchild and Mike G. for these additional insights.

Every little bit helps me understand how this thing really works, so I can be smarter in setting it up and enjoying it.

I originally asked the question "So How Does an iPod Actually Work?"  And you guys are giving me just what I wanted. Thank you, thank you.

So How Does an iPod Actually Work?

Reply #32 –
Quote from: Mike Giacomelli on
Quote from: kornchild2002 on


The OS (firmware) might be stored on the hard drive but it is loaded onto the RAM while running.  I found that the 5G iPod has a 1MB (or 512KB) NOR flash ROM chip that contains the bootloader program that tells the iPod to load the OS from the hard drive.  A small portion of RAM is used to hold the iPod OS once it is loaded.  After a 5G/5.5G iPod is turned off for a period of 24 hours, all information is deleted from the RAM thus causing the iPod to perform a quick boot.  If the iPod is turned off then turned back on in less than a 24 hour period, the OS is already loaded from the RAM thus making boot times instant.


It doesn't directly reboot.  It actually dumps it's state to the disk and then powers down to save battery.  Presumably they included the real time clock so that they'd have something to trigger the power off interrupt.


In this aspect the 5G must differ from the 4G, as mine (a 4G) performs a complete reboot and returns to the main menu (forgetting what song I was listening to etc.) if i leave it for 24 hours (actually I think its closer to 3 days for the 4G, that's just a guess)

So How Does an iPod Actually Work?

Reply #33 –
Quote from: david_dl on
Quote from: Mike Giacomelli on
Quote from: kornchild2002 on


The OS (firmware) might be stored on the hard drive but it is loaded onto the RAM while running.  I found that the 5G iPod has a 1MB (or 512KB) NOR flash ROM chip that contains the bootloader program that tells the iPod to load the OS from the hard drive.  A small portion of RAM is used to hold the iPod OS once it is loaded.  After a 5G/5.5G iPod is turned off for a period of 24 hours, all information is deleted from the RAM thus causing the iPod to perform a quick boot.  If the iPod is turned off then turned back on in less than a 24 hour period, the OS is already loaded from the RAM thus making boot times instant.


It doesn't directly reboot.  It actually dumps it's state to the disk and then powers down to save battery.  Presumably they included the real time clock so that they'd have something to trigger the power off interrupt.


In this aspect the 5G must differ from the 4G, as mine (a 4G) performs a complete reboot and returns to the main menu (forgetting what song I was listening to etc.) if i leave it for 24 hours (actually I think its closer to 3 days for the 4G, that's just a guess)


You misunderstand me.  Your Ipod doesn't perform a reboot after a fixed amount of time, rather it actually switches off completely, powering down the DRAM and erasing it's contents.  Then, when you next go to use it, it boots up. 

Basically, think of the difference between what happens when you chose "shutdown" in windows verses "restart".  Although both do force the system to reload from the disk, shutdown has the added benefit of not consuming power while the device is not being used, which is the whole point of the feature.

So How Does an iPod Actually Work?

Reply #34 –
Quote from: Mike Giacomelli on
Quote from: david_dl on

Quote from: Mike Giacomelli on
Quote from: kornchild2002 on


The OS (firmware) might be stored on the hard drive but it is loaded onto the RAM while running.  I found that the 5G iPod has a 1MB (or 512KB) NOR flash ROM chip that contains the bootloader program that tells the iPod to load the OS from the hard drive.  A small portion of RAM is used to hold the iPod OS once it is loaded.  After a 5G/5.5G iPod is turned off for a period of 24 hours, all information is deleted from the RAM thus causing the iPod to perform a quick boot.  If the iPod is turned off then turned back on in less than a 24 hour period, the OS is already loaded from the RAM thus making boot times instant.


It doesn't directly reboot.  It actually dumps it's state to the disk and then powers down to save battery.  Presumably they included the real time clock so that they'd have something to trigger the power off interrupt.


In this aspect the 5G must differ from the 4G, as mine (a 4G) performs a complete reboot and returns to the main menu (forgetting what song I was listening to etc.) if i leave it for 24 hours (actually I think its closer to 3 days for the 4G, that's just a guess)


You misunderstand me.  Your Ipod doesn't perform a reboot after a fixed amount of time, rather it actually switches off completely, powering down the DRAM and erasing it's contents.  Then, when you next go to use it, it boots up. 

Basically, think of the difference between what happens when you chose "shutdown" in windows verses "restart".  Although both do force the system to reload from the disk, shutdown has the added benefit of not consuming power while the device is not being used, which is the whole point of the feature.



Sorry, that is what I meant, i should have been more clear. I used the term reboot to indicate that it was booting up from scratch. But my 4g does not remember state after doing this, which you claim your 5G does.