G is a dummy index register whose value is alway zero, and hence causes the offsets to be treated as absolute addresses in the zeroth ( global ) segment . ( Incidentally, since X3 is the standard index register, the assembler program allows', X3, address'to be abbreviated to', address'.)
22.
For either, the BIOS reads the first physical sector of the media into RAM at absolute address 7C00 hex, checks the signature in the last two bytes of the loaded sector, and then, if the correct signature is found, transfers control to the first byte of the sector with a jump ( JMP ) instruction.
23.
Like them, jalr can be used with the instructions which set the upper 20 bits of a base register in order to make 32-bit branches, either to an absolute address ( using lui ) or a PC-relative one ( using auipc for position-independent code ) . ( Using a constant zero base address allows single-instruction calls to a small ( the offset ), fixed positive or negative address .)
