Questions about computer memory

As we all know, when the computer is started, the BIOS is first executed from the physical address 0xffff0.ProgramWhile the BIOS program is solidified in the ROM

So I thought this address should not belong to the memory stick. Further, if the BIOS Rom size is 64 K, it should occupy f000: 0 ~ F000: FFFF address; correspondingly, the video memory BIOS may occupy e0000 ~ Bffff address space, and some other BIOS will occupy some address space. If ROM and RAM are not related to each other, but logically group the memory space of the computer, you can explain the two points:

1. Writing data to Rom is invalid. For example, mov [f000: 1], ax (the command is not necessarily correct, so it is convenient to explain the problem), this command will not change the f0001 unit; MoV [b800: 0], ax writes data to the video memory without passing through the memory, because this address is stored in the video card, and there is no half-cent relationship with the memory.

2. CPU-accessible storage units (ROM and RAM) are connected to the address bus. The layout of the address bus on the motherboard determines the size of the video card that the motherboard can accept, you cannot change the larger video card on the same motherboard. In addition, 1 GB memory is not a continuous storage unit, which is separated by various Rom

The above two points should be used for experiment demonstration. Unfortunately, I did not do it. I hope some experienced friends will tell me that I really want to. Thank you ~

However, I wrote data to the BIOS Rom address space under the built-in dos tool of Windows 7. At that time, I could write data. Later I thought about how to run the DOS in v86 mode. The address in this mode is not a real physical address, so it cannot prove anything.

Later, I learned about BIOS ing technology, that is, the system maps various BIOS addresses to the memory, and f0000 ~ Fffff and other addresses are still in the memory, but these address units cannot be modified .. So does this refute the above conjecture. But there are still some doubts. In this case, the 32 IP Address Bus is connected to the memory, and all the programs in the Rom are finally executed in the memory through the ing technology. But another problem is that there is no address bus. How can the CPU access the Rom? Even if the ing technology is used, you have to access the ROM first.

There is also a conjecture that 32 address bus connects both Rom address space and Memory RAM. However, there are still doubts about how the CPU can access the ROM or Memory RAM at the same address, and the computer never permits ambiguity.

I personally agree with the first conjecture. I have heard from my friends that I have bought two memory sticks (4 GB in total), but the operating system can only recognize 2.9 GB, the possible cause is that although there are 4 GB memory, it can only occupy a small amount of address space, and other address spaces are occupied by various Rom

If you have any questions, please kindly advise me. Thank you first :)