Tracing instruction execution from startup

Question

I have somewhat of a pipe dream to program my own OS. While that will probably never be realized, I figure it might be fun and educational to at least work toward that goal. And I figured the best way to do that would be to start with figuring out what my own computer does. To that end, I've downloaded the program RW-Everything, which shows me what is in memory, and the AMD Processor Programmer's Reference Manual (all 5 volumes). My problem, essentially, is that what RW-Everything says my computer does doesn't appear to match what the Manual says should be done. So I would like to know who or what is wrong.

According to the manual, the processor starts at FFFFFFF0. The instructions here are

90 (nop) 90 (nop) E9 23 F6 (jmp near -09DD) which goes to FFFFFFF5 - 9DD = FFFFF618

Now at FFFFF618 the instructions are FA (cli) 30 C0 (xor al, al) E6 80 (out 80, al) 66 8B E0 (mov esp, eax) 66 8B EA (mov ebp, edx) 66 BB 80 FD FF FF (mov ebx, FFFFFD80) 66 2E 0F 01 17 (lgdt cs:[edi])

Now the problem here is that the cs base address is still at its initial value of FFFF0000 and edi is still at its initial value of 0. So the GDTR should be loaded with 6 bytes starting at FFFF0000. But at this location is a sea of FF's, meaning that the GDT base address is set to FFFFFFFF, which is a highly unlikely address at which to begin the GDT. So...what gives? Did I make a mistake somewhere?

Answer 1

You disassembled at FFFFF618h and found these instructions:

FA                (cli)
30 C0             (xor al, al)
E6 80             (out 80, al)
66 8B E0          (mov esp, eax)
66 8B EA          (mov ebp, edx)
66 BB 80 FD FF FF (mov ebx, FFFFFD80)
66 2E 0F 01 17    (lgdt cs:[edi])

Did I make a mistake somewhere?

Your last line is wrong. Because at the time of execution the CPU is still running in the Real Address Mode and the instruction was not encoded with an explicit Address Size Prefix (a byte 67h), the correct translation is:

lgdt [cs:bx]   ;An R/M field of 111 denotes [BX] in 16-bit addressing.

Now the BX register was initialized at 0FD80h, so you should take a look there!