Memory Addresses!

[nadirshakur]

It says in my A+ book that the OS assigns memory addresses like cells. But it also says that Memory address range F is reserved for the BIOS and that the CPU turns to it right as your turn on the computer. If the OS is the one that assigns these address numbers how does the BIOS take the f range before the OS loads so that CPU can access BIOS?

I do not understand the concept behind memory addresses and the memory bus. Is the memory bus same as the front side bus. Doesn't the front side bus have different purposes like IRQ lines, I/O lines etc, why is it the same as memory addresses if infact it is?

 

[Peter]

Some devices are enabled to decode right from out of power-on reset. The uppermost 64KBytes of BIOS ROM are the most obvious thing in this category - because just like you figured out, there HAS to be executable code where the CPU puts the needle on the record.

That initial piece of code (the "bootblock" in PC-speak) then goes to detect and enable the RAM, unpack the main part of BIOS (the "POST") into RAM, and jump there. BIOS POST (not the OS!) then enables everything else using PnP procedures to map everything to useful addresses, readies the runtime APIs, and then finally attempts to load (and jump into) a valid boot sector from mass storage.

 

[BrownTown]

don't feed him and he might go away...

 

[nadirshakur]

Peter I think I understand. Do you know about shadowing RAM? I read that shadowing RAM is copying the ROM BIOs to RAM for faster reads. If the ROM programs are executed directly from ROM, memory addresses are assigned to this ROM. If the programs in BIOS are "shadowed" first, then the same memory addresses are assigned to this area of RAM. In this case what the hell is the difference between these two states? I do not understand the difference between "memory address" and "this area of RAM".
Thank you.

 

[Peter]

ROM is copied to RAM in the exact same memory location, the ROM then disabled to expose the RAM, and the RAM write-protected to make it look like ROM - only faster.

 

[piasabird]

There are also memory addresses on the CPU itself called Registers. Then there is Cache Memory which may be attached to the CPU closer than RAM and faster also. Thee are Memory settings in the BIOS. Even the BIOS is a kind of memory.

So what is the difference between the CMOS and the BIOS? We kind of lump them together. On a mainframe you have to have new machine code just to access an extra hard drive (Sometimes). The PC has this built in so you dont have to write machine code for every different motherboard and every different drive. This is like built into the chipset.

The operating system tells the sofware how to interact with the hardware. This is a little simplistic, because even that has subcategories. Lookup things like Kernal, Kernel, however it is spelled. Then there is IRQ Assignment for Interrupt Requests kind of like a priority Que. Then there is the evil DMA Dynamic Memory Access. Part of your CPU starts accessing memory and then sends a signal back when it gets a certain amount in the right place. In the meantime, your CPU works on a few other things all at once.

Darn computers are so complicated.

 

[nadirshakur]

I am still not getting. I appreiciate your patience but maybe we can take one step at a time. Memory is volatile RAM. Meaning it needs constant flow of electricity to retain its data. When a computer is shutdown there is nothing in there not even memory addresses. When a computer turns on the OS assigns these addresses. And if you even check the resource manager you will see that the F range is reserved for BIOS. Its a standard manufacturers agreed on. My question is if it is indeed the OS that assigns memory addresses, how does that CPU read memory addresses before the OS is load and I was just using the BIOS as an example as there are many other procedures that occur before the BIOS turns to read the CMOS configuration to look for a MBR program.

 

[Peter]

Told you. ROM is memory. Chips have hardwired properties to provide a starting point. The BIOS ROM's poweron starting point is such that the CPU sees it. End of.

 

[Steve]

Memory is not volatile RAM. RAM is volatile memory. ROM (Read Only Memory) such as the BIOS or any firmware chip, and NVRAM (Non-Volatile RAM, your CMOS chip) maintain their programming between power states.

 

[Peter]

... and the system's core chipset hard enables access to that ROM right upon powerup.

 

[nadirshakur]

ok

 

[bendixG15]

Did you give up ?..

 

[nadirshakur]

no i understood, i didnt realize that it means 1048576 memory addresses not bits.

 

[skimple]

Originally posted by: Peter

where the CPU puts the needle on the racket.

Did you mean "put the needle on the record?

(and dance, dance, dance)

 

[insename2]

Originally posted by: sm8000
Memory is not volatile RAM. RAM is volatile memory. ROM (Read Only Memory) such as the BIOS or any firmware chip, and NVRAM (Non-Volatile RAM, your CMOS chip) maintain their programming between power states.

cmos is nvram? i thought the battery kept it alive...

 

[insename2]

yea... that makes no sense... what happens to ur bios settings if u pop the battery out for a minute? it gets reset... it does maintain its data but its cause theres a battery keeping it alive..

 

[Steve]

Originally posted by: insename2

Originally posted by: sm8000
Memory is not volatile RAM. RAM is volatile memory. ROM (Read Only Memory) such as the BIOS or any firmware chip, and NVRAM (Non-Volatile RAM, your CMOS chip) maintain their programming between power states.

cmos is nvram? i thought the battery kept it alive...

The battery allows the CMOS to retain the settings you adjust (via BIOS) while the PC is off.

 

[Peter]

Originally posted by: skimpleDid you mean "put the needle on the record?

(and dance, dance, dance)

Yes of course I meant that *shakes fist at auto-correction*

 

[Peter]

Originally posted by: insename2
yea... that makes no sense... what happens to ur bios settings if u pop the battery out for a minute? it gets reset... it does maintain its data but its cause theres a battery keeping it alive..

The term "non-volatile" is used for memory that retains its contents under normal circumstances - and that includes battery-backed storage.

 

[nadirshakur]

wait a minute, isnt volatile RAM, RAM that needs a constant source of power to keep its data like ur normal system memory?

Originally posted by: sm8000

The battery allows the CMOS "to retain the settings" you adjust (via BIOS) while the PC is off.

 

[Varun]

Originally posted by: nadirshakur

wait a minute, isnt volatile RAM, RAM that needs a constant source of power to keep its data like ur normal system memory?

Originally posted by: sm8000

The battery allows the CMOS "to retain the settings" you adjust (via BIOS) while the PC is off.

Yes, that is why the BIOS uses NV (NON-Volatile) RAM. The BIOS has some memory that is volatile, and that stores your settings that you can make (ie memory speed, hard drive assignment, bus speed etc). This memory is kept alive by a battery on the motherboard. When you pull that battery you will loose your temporary settings.

The main BIOS code is in flash memory, that does not lose it's memory when powered off (think thumb drive). This is the code that starts the computer BEFORE the OS is even looked at. Once the computer is booted, it accesses the hard disk and boots the operating system.

So when you flash your BIOS, you erase and rewrite the code located in the BIOS, which is then locked in and won't be lost until you flash it again.

On another topic, I find it interesting that the BIOS is also called the CMOS. CMOS is a type of transistor (complementary metal-oxide-semiconductor), of which the BIOS is made of. All integrated circuits in a computer are CMOS as well though (CPU, chipset, HDD controller, graphics card) so I just wonder why the term CMOS got stuck with the BIOS.

 

[CrispyFried]

not 100% sure on this, but here goes: back in the IBM AT (80286) days cmos chips were the original low power chip that could use a battery to retain data, but it was slow compared to TTL (power hungry) chips.. so the cmos chips were only used for the bios data that was only needed once at boot up to tell the bios what HD was installed, the time and date etc. the speedy and power hungry TTL chips ran the rest of the computer.

so the name has stuck ever since, even though cmos tech has come a long way since.

 

[Varun]

Cool thanks I always wondered why a BIOS was called CMOS. That actually makes a lot of sense.

 

[nadirshakur]

BIOS isnt CMOS but I think I know what you mean. I am not going to bother to ask I will probably just raise your blood pressure levels and still not get anywhere.

 

[Varun]

well, the BIOS is built of CMOS, every chip in the world is now for the most part, since TTL is so power hungry...

But you know when people say CMOS in regards to a computer the generally mean the BIOS. That's what I mean anyways.