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LoadingMSDOS
When a pc starts program execution begins at address 0x0FFFF0H. Before this all registers are blanked and the processor is brought into a reset state. This address corresponds to ROM and usually contains a jump instruction to system test code and the ROM bootstrap routine. The ROM bootstrap routine loads the disk bootstrap routine into memory. The disk bootstrap routine checks for a copy of MS-DOS and starts executing it. It does this by reading the first sector of root directory. It then searches for IO.SYS first and then MSDOS.SYS.These are read into memory. IO.SYS contains hardware specific code that is needed only at the startup. It also contains a module called SYSINIT which is called by manufacturer's BIOS initialization code and deal with memory. Then SYSINIT code is overlayed by MSDOS.SYS. The initialization code in MSDOS.SYS initializes internal tables and work areas and sets up the interrupt vectors (20H thru 2FH). Then it initializes all resident device drivers. Too few details are given here for a detailed reference do read 'Advanced MS-DOS Programming' by Ray Duncan.
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LoadingMSDOS
How MS-DOS is Loaded
When a pc starts program execution begins at address 0x0FFFF0H. Before this all registers are blanked and the processor is brought into a reset state. This address corresponds to ROM and usually contains a jump instruction to system test code and the ROM bootstrap routine. The ROM bootstrap routine loads the disk bootstrap routine into memory. The disk bootstrap routine checks for a copy of MS-DOS and starts executing it. It does this by reading the first sector of root directory. It then searches for IO.SYS first and then MSDOS.SYS.These are read into memory. IO.SYS contains hardware specific code that is needed only at the startup. It also contains a module called SYSINIT which is called by manufacturer's BIOS initialization code and deal with memory. Then SYSINIT code is overlayed by MSDOS.SYS. The initialization code in MSDOS.SYS initializes internal tables and work areas and sets up the interrupt vectors (20H thru 2FH). Then it initializes all resident device drivers. Too few details are given here for a detailed reference do read 'Advanced MS-DOS Programming' by Ray Duncan.
back to BootLoaders
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