x86 - Why is this line of assembly an 'invalid use of register'? -


i going through os development tutorials , see following section of code:

.intel_syntax noprefix  do_e820:     xor ebx, ebx        # ebx must 0 start     xor bp, bp      # keep entry count in bp     mov edx, 0x0534d4150    # place "smap" edx     mov eax, 0xe820     mov [es:di + 20], dword 1   # <<<this line don't     mov ecx, 24     # ask 24 bytes     int 0x15     jc short .failed    # carry set on first call means "unsupported function"     mov edx, 0x0534d4150    # bioses apparently trash register?     cmp eax, edx        # on success, eax must have been reset "smap"     jne short .failed     test ebx, ebx       # ebx = 0 implies list 1 entry long (worthless)     je short .failed     jmp short .jmpin

when try assemble home-rolled kernel, assembler complains error: invalid use of register @ line have marked. if helps understand situation, using gcc i686-elf cross assembler.

the use of 16-bit registers inside otherwise 32 bit code suggests snippet of ancient "magic" code, passed on father son in eldritch rituals. during time, supported syntax of assemblers changed, , once valid line no longer accepted modern assembler using.

this line

mov [es:di + 20], dword 1

is more commonly written as

mov dword ptr es:[di + 20], 1

which notation used intel in manuals.


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