What was CS register's value at boot time?

Question

I'm a green hand to assembly language. These days I used bochs to simulate the boot procedure from virtual floppy. But one thing I don't know is what the CS register value is when the system initializes.

;;  init registers

org 0x7c00

BaseOfStack equ 0x7c00

Label_Start:

mov ax, cs
mov ds, ax
mov es, ax
mov ss, ax
mov sp, BaseOfStack

;; clear screen

mov ax, 0600h
mov bx, 0700h
mov cx, 0
mov dx, 0184h
int 10h

The above code is only part of it. When the ORG command was executed, what was the CS register value at the instruction mov ax, cs. Is it the same as 0x7c00? Thanks.

Below is bochsrc configuration:

romimage: file="$BXSHARE/BIOS-bochs-latest"
vgaromimage: file="$BXSHARE/VGABIOS-lgpl-latest"
boot: floppy
floppy_bootsig_check: disabled=0
floppya: type=1_44, 1_44="myboot.img", status=inserted
pci: enabled=1, chipset=i440fx
vga: extension=vbe, update_freq=15
cpu: ips=15000000
log: bochsout.txt
mouse: enabled=0
megs: 32

Answer 1

The specification says that CS:IP = 0000:7C00 at boot time, but some BIOS vendors boot off 07C0:0000 instead. The best way is to write your boot sector such that it works with both conventions by doing a far jump to a known selector early on:

    org 0x7c00

    xor ax, ax
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7c00
    jmp 0x0000:set_cs

set_cs:
    ...