What are the different areas of Memory & Disk?

Question

I'm neither sure about if this is a right place to ask nor sure about how to put my query.

Let me put it this way:

• Main Memory starting at 0x00000 to 0xFFFFF.
• Diskspace starting at 0x00000000 to 0xFFFFFFFF.

But what we'll be able to access will not be from 0th byte till last byte right? On hardisk I guess at the 0th byte we have MBR. & at someplace we have Filesystem (we are able to acess only this). What else?

Similarly with the Main memory. We have some Kernel Memory & User Memory(in which each processes live). What else?

My question is what are all the regions from 0th byte till the last byte? I don't know what to search for or where to find such information? If any one can post some links, that would be great.

EDIT:

I'm using x86 32Bit on Windows. Actually I was reading a book on Computer security where author mentions that a malware can either live on the disk or in the memory.(which is very true). But when we say computer is infected that doesn't mean only files (which are part of filesystem) is infected. There are other area's which are not mean't for user, like MBR. or Kernel Memory.

So, the question popped up in my mind. What are all such areas that I may not be aware about?

Answer 1

Apart from the fact that the answer to this question is highly dependent on the OS, disk space is not at all part of the main memory. On Intel architectures, disk access takes some I/O address space (which is different from memory address) per channel. And the exact number of words depends on what channel: IDE/ATA/SATA/SCSI. On other architectures which are memory mapped like the PowerPC disk access do take some memory address space, but still not much.

To illustrate (and be warned that this is a very simplified example, not the real world), assume a memory mapped CPU* like the PowerPC trying to access a disk with LBA addressing. The disk really only need 2 to 3 words of memory to hold multiple Gigabytes of data. That is, we only need 12 bytes to store and retrieve Gigabytes of data:

• 2 words (8 bytes) to tell the disk where to seek to, that is, at what address do we want to read form or write to.
• 1 word (4 bytes) to actually do the read and write. Every time you read from this address, the 2 word pointer automagically increment by 1 character (or 4 if you read in 32 bits).

But the above is an abstracted view of what really happens. Most disk controllers have several more registers to control power management, disk spin speed, enter and exit sleep modes etc.

And what are the addresses of these memory locations? Well, it depends on what I/O channel you're talking about. The old-school ISA bus depends on the user setting jumpers on cards to set the addresses. So for those you need to ask the user. The PCI bus auto-negotiates the addresses with the disk controllers at boot time and then, depending on architecture, either tells your bios what devices exist or pass them as parameters to the bootloader or store them in some temporary registers on the system bus. USB works like PCI but negotiates with the OS instead of the BIOS... etc.

As you can see, there is no simple answer to this even if you limit it to only specific cases like Windows7 running on 64 bit AMD CPU running on Dell motherboards.

*note: since you're worried about memory locations.

Answer 2

Your question is complex, and hard to answer without knowing the scope of where the view of memory is.

Pretending we're in ring-0 with direct mapped memory, a PC-compatible has multiple memory regions. Lower memory, BIOS mapped code, IO ports, video memory, etc. They all live in the same memory space. You communicate with peripherals by reading and writing from specific memory addresses (which are mapped to those components). These addresses are setup by the hardware in question and the drivers in use.

Once we enter user mode, you have to deal with virtual memory. Addresses are symbolic, and may or may not map to any particular part of physical memory. I'd suggest reading up on Virtual memory