Inline Assembly Jump Error

Question

Why does this fail, once Masm reaches jmp?

struct gdt_entry
{
    unsigned short limit_low;
    unsigned short base_low;
    unsigned char base_middle;
    u         


        

Answer 1

Try to put the following pragma before and after the structure definitions:

#pragma pack(push,1)

struct gdt_entry
{
    unsigned short limit_low;
    unsigned short base_low;
    unsigned char base_middle;
    unsigned char access;
    unsigned char granularity;
    unsigned char base_high;
};

struct gdt_ptr
{
    unsigned short limit;
    unsigned int base;
};

#pragma pack(pop)

Although it has no effect on gdt_entry, these instructions change the memory layout of the gdt_ptr structure. The default behavior of the compiler is to align structures elements on 32 bits. Hence, the previous definition would be equivalent to :

struct gdt_ptr
{
    unsigned short limit;
    unsigned short unused;
    unsigned int base;
};

which is invalid from the processor point of view.