What is significance of memory at 0000:7c00 to booting sequence?
The simple answer is that 7C00h is 1k (512 bytes for the boot sector plus an additional 512 bytes for possible boot sector use) from the bottom of the original 32k installed memory.
The happy answer is that org 7C00h has become synonymous with boot sector - boot loader programming.
The ":" is a holdover from segmented memory days, when PCs ran in real mode and could only do 64K at a time. The number to the left of the ":" is your segment, the number to the right is your address.
The windows debug command accepts this notation if you want to poke around in memory yourself:
C:\Users\Seth> debug
-d0000:7c00
0000:7C00 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000:7C10 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000:7C20 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000:7C30 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000:7C40 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000:7C50 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000:7C60 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
0000:7C70 00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00 ................
With regard to this particular address, it's just an address that was picked to load the MBR, See: https://web.archive.org/web/20140701052540/http://www.ata-atapi.com/hiwmbr.html
"If an MBR is found it is read into memory at location 0000:7c00 and INT 19 jumps to memory location 0000:7c00"
Read this article:
http://en.wikibooks.org/wiki/X86_Assembly/Bootloaders
From the above URL, BIOS (which is effectively PC hardware) will make the jump to memory at 0000:7c00 to continue execution in 16-bit mode.
And to quote from above:
A bootloader runs under certain conditions that the programmer must appreciate in order to make a successful bootloader. The following pertains to bootloaders initiated by the PC BIOS:
- The first sector of a drive contains its boot loader.
- One sector is 512 bytes — the last two bytes of which must be 0xAA55 (i.e. 0x55 followed by 0xAA), or else the BIOS will treat the drive as unbootable.
- If everything is in order, said first sector will be placed at RAM address 0000:7C00, and the BIOS's role is over as it transfers control to 0000:7C00. (I.e. it JMPs to that address)
So from bootup, if u want the CPU to start executing your code, it has to be located in memory at 0000:7c00. And this part of the code is loaded from the first sector the harddisk - also done by hardware. And it is only the first sector which is loaded, the remaining of other parts of the code then have to be loaded by this initial "bootloader".
More information on harddisk's first sector and the 7c00 design:
http://www.ata-atapi.com/hiwdos.html
http://www.ata-atapi.com/hiwmbr.html
Please don't confuse with the starting up mode of the CPU - the first instruction it will fetch and execute is at physical address 0xfffffff0 (see page 9-5):
http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-vol-3a-part-1-manual.pdf
and at this stage it is executing non-volatile (meaning you cannot reprogram it easily, and thus not part of bootloader's responsibility) BIOS code.