RAM below 1MB is...?

[OdinVex]

RAM below the 1MB extended memory is what? Internal/embedded? Where is it located? Kind is it? Is it borrowed from Extended RAM and used that way? What speed is it? Latencies anyone? Never bothered to think about it until now, hm.

[trinopoty]

The "RAM" below 1MB is the RAM installed on your PC.

[OdinVex]

Meaning the physical chips or some soldered RAM (like SPI flash etc)?

[Brendan]

Hi,

Meaning the physical chips or some soldered RAM (like SPI flash etc)?

Meaning that all RAM in the entire computer is normally from the same RAM modules and that RAM below 1 MiB and RAM above 1 MiB is identical because it's the exact same chips providing both.

The only thing "special" about the RAM below 1 MiB is that it can be accessed in real mode.


Cheers,

Brendan

[OdinVex]

So the BIOS/(UEFI???) first runs off what, before setting up RAM? CPU'd probably be like ARM but BIOS, not sure.

*Edit* Trinopoty, nice avatar, good capture.

[Brendan]

Hi,

So the BIOS/(UEFI???) first runs off what, before setting up RAM? CPU'd probably be like ARM but BIOS, not sure.

For modern 80x86; firmware is in some sort of ROM (e.g. flash ROM) just below 0x100000000. At power on it initialises things (including the RAM controller and detecting RAM sizes, etc). After that, for "PC BIOS" it typically copies part of itself to RAM just below 0x00100000 and tells the chipset to pretend that area is "read only", then goes looking for something to boot.


Cheers,

Brendan

[OdinVex]

*Edit [This was answered]

[Combuster]

If you read all the hints, there's no silicon difference between "base" and "extended" RAM - in fact all of it is there on the same chip. There's no logic other than checking if an address should be routed to the DRAM chip or to one of the IO buses. That check is baked in silicon and doesn't even take half a clock cycle to run, let alone be so stupid to compare it against each address in turn.

[OdinVex]

I read again what Brendan said and it makes sense, I must have been tired.