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You wrote:Unfortunately, this is not available in long mode.

• Intel added a REX-based 64-bit jump for EM64T. This is technically incompatible, but unlikely to have any effect (Who would code a 32-bit far jump with a REX prefix?) (Edit: This is the case)
• Intel didn't and your emulator is broken

.global main
main:
        mov %cs, %bx
        mov %bx, jcs
        movabs $jmpbuf, %rax
        rex.w ljmp  *(%rax)

.data
jmpbuf: .quad after
jcs:    .word 0
        

.text
after:
        mov $0, %rax
        ret

• On AMD64, this code crashes with a SIGSEGV (The processor loads the CS from part way through the offset and therefore gets 0000h and crashes)
• On Intel64, this code works
• On AMD64, this code works if the .quad offset is changed to a .long, obviously with limited addressing
• On both, this code works if the .quad offset is changed to a .long and the rex.w prefix removed

Owen wrote:far return also works to my knowledge (Checking the AMD manuals, it doesn't say anywhere it shouldn't)

gerryg400 wrote: ...

- gerryg400

proxy wrote:traditionally in protected mode, after my lgdt, I would do something like (from memory, might be not exactly right)

Code: Select all

ljmp $0x08, 1f; 1:

. Unfortunately, this is not available in long mode. So what is the correct way to do that? I've tried something like this with no success (bochs shows an exception on the ljmp instruction).

    lgdt [GDT.Pointer]
    mov rbp, rsp
    push QWORD GDT.KData
    push rbp
    pushfq
    pushfq
    pop rax
    and rax, 1111111111111111111111111111111111111111111111101011111011111111b
    push rax
    popfq
    push QWORD GDT.KCode
    push QWORD .Flush
    iretq
    
.Flush:
    mov ax, GDT.KData
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

Owen wrote:Of course, there is pretty much no reason to reload the GDT in long mode.

StephanVanSchaik wrote:

Owen wrote:Of course, there is pretty much no reason to reload the GDT in long mode.

There are reasons to reload the GDT. First of all, the GDT you initially get from boot loaders isn't reliable, since you usually don't know where it is and what descriptors it has. Second of all, after enabling long mode your GDT will not be the most-ideal setup at all so you will want to reload the GDT with one that has a supervisor code entry, a supervisor data entry, a user code entry, a user data entry and a TSS entry or whatever entries you want to have in your GDT (in which case the two different data segments are actually not playing a role and are only there to remain "compatible" with 32-bit mode, since the data descriptors are almost entirely ignored).


Regards,
Stephan J.R. van Schaik.

.global main
main:
        mov %cs, %ax
        movzx %ax, %rax
        pushq %ax
        pushq $next
        lretq

next:   mov $str, %rdi
        jmp puts

.section .rodata
str:    .asciz "Hello world!"

// Switch code segment
// void switchseg(short newseg)
.global switchseg
switchseg:
    pop %rsi
    push %rdi
    push %rsi
    lretq

Owen wrote:By the way: far returns work to switch segments, at least according to my limited (userspace) testing:

Code: Select all

.global main
main:
        mov %cs, %ax
        movzx %ax, %rax
        pushq %ax
        pushq $next
        lretq

next:   mov $str, %rdi
        jmp puts

.section .rodata
str:    .asciz "Hello world!"

Runs successfully. One could create the following:

Code: Select all

// Switch code segment
// void switchseg(short newseg)
.global switchseg
switchseg:
    pop %rsi
    push %rdi
    push %rsi
    lretq

According to AMD64 Architecture Programmer's Manual Volume 2: System Programming, section 2.5.10, it is illegal to use the far jump instruction in 64-bit mode.

I did a bit of experimentation to check this was not a documentation oversight on either's part. First, the code:

Code: Select all

Code:
.global main
main:
        mov %cs, %bx
        mov %bx, jcs
        movabs $jmpbuf, %rax
        rex.w ljmp  *(%rax)

.data
jmpbuf: .quad after
jcs:    .word 0
        

.text
after:
        mov $0, %rax
        ret

Experimental results:
On AMD64, this code crashes with a SIGSEGV (The processor loads the CS from part way through the offset and therefore gets 0000h and crashes)
On Intel64, this code works
On AMD64, this code works if the .quad offset is changed to a .long, obviously with limited addressing
On both, this code works if the .quad offset is changed to a .long and the rex.w prefix removed

This is consistent with both's documentation

gerryg400 wrote: Owen, did you have the NULL selector ? Or a valid selector in jcs?

- gerryg400

        mov %cs, %bx
        mov %bx, jcs