On Fri, 2010-01-08 at 09:44 +0800, Qing Wang wrote: > Thank Andrew. > For how to change modes, it is OK because we can load new gdt/cs/ds/..., change MSR_EFER and stack, and do long jump. > For how to link two different versions of libc, I don't know why I have this issue. I write my code, even including early_printk().I don't call any library. In all c files, I only include my .h files without any .h files in the library. I don't know all the detail, but I believe that if your program has a main function, you are using libc. > For the fact a binary is either 32- or 64-bit, I maybe make you confused. I admit we can't link a 32-bit object with a 64-bit object. I just want to implement the same functionality of ".code32" and ".code64" in assembly. > As you know, I can define a section in assembly as 32-bit code, then the compiler (or GAS?) will regard the part as 32 bit. Likewise, I also can define a section as 64-bit code in the same assembly file. > For example, in the same assembly file, we can write as follows. > ENTRY(entry32) .code32 mov $8, %eax .code64 mov $8, %rax > Then when running, the first "mov" will be regarded as 32 bit instruction and the second as 64-bit. You don't need to change modes in this situation. Here's an example: GAS LISTING mix.s page 1 1 # mix.s 2 # Mix 32-bit and 64-bit. 3 # Bob Plantz - 7 Jan. 2010 4 .text 5 .globl main 6 main: 7 0000 55 pushq %rbp # save caller's base pointer 8 0001 4889E5 movq %rsp, %rbp # establish our base pointer 9 10 0004 4889C3 movq %rax, %rbx # 64-bit 11 0007 89C3 movl %eax, %ebx # 32-bit 12 0009 6689C3 movw %ax, %bx # 16-bit 13 14 000c 88E0 movb %ah, %al # 8 bits in 32-bit mode 15 000e 4188C0 movb %al, %r8b # 8 bits in 64-bit mode 16 17 0011 B8000000 movl $0, %eax # return 0 to caller 17 00 18 0016 4889EC movq %rbp, %rsp # restore stack pointer 19 0019 5D popq %rbp # restore caller's base pointer 20 001a C3 ret # back to caller Notice that most of the mov instructions have a REX prefix (41 or 48). That's because they use 64-bit mode features. The mov instructions on lines 11, 12, 14, and 17 do not use 64-bit mode features. The CPU is in 64-bit mode when it executes this code, but these instructions are exactly the same as in 32-bit mode. The pushq and popq instructions do not need a REX prefix because the CPU is in 64-bit mode. They treat the stack in 64-bit units. I still don't understand why you don't simply recompile all your C code on a 64-bit system. Then it will all be 64-bit. --Bob
