The computer motherboard is Inventec’s B810G, which finally solved the problem after much trouble.
INT 19 loads 512 bytes of MBR into memory 0x7c00, then jumps to 0x7c00 to execute the MBR executable (master booter). Master booter at least needs to do these things:
checks whether MAGIC (Signature) is a valid value (hexadecimal 55 AA);
moves itself to another location (usually 0x0600), leaving 0x7C00 to 0x7C00 +512K, in case a boot sector program is subsequently loaded at that location. This is the same as loading a boot sector program directly from a diskette. According to the design, it can be moved to any non-conflict position (that is, not reserved for other programs). But in general, one end space is found between 0X000800 and 0X0A0000.
view the partition table. Load 0X7C00 at the first Sector of the partition that is set to be active. Normally, this Sector will be the boot Sector. Finally, the master booter jumps to 0X7C00 and executes boot sector.
Gate A20 Option
function: set the control mode of A20 address line
set value :Fast or Normal
A20 address line control mode refers to how the system accesses the part of the memory space above 1MB. Because of history, early computers had only 1MB of memory, and software could use up to 1MB. Later, due to technological advances, software required more memory, so the system was required to provide more memory space. The A20 signal was created to solve this problem.
when this option is set to “Normal”, the system USES the keyboard controller chip to control the A20 signal; When this option is set to “Fast”, the system USES a chip on the motherboard to control the A20 signal. When using the motherboard chip to control the A20 signal, the memory access speed can be improved, thus improving the performance of the system. Therefore, it is recommended to set this option to “Fast”.