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what is the error

what is the error , and can i get programme like debug in DOS for Linux ?

SmartBoy wrote: but when i try to boot from floppy it's show error for me :-\

msg db "Hello, World!",0

Ryu wrote: Well as I take it, the jmp short followed by a nop is typically found in FAT12 which follows the BPB. I really don't think the system BIOS will parse the BPB in anyway, nor should it take the address of a short jmp and jump directly there. The standard is that it should jmp to linear address 7C00h but no standard that I know of that specifies that CS should be at a particular segment apon entering bootsector. The BIOS should also by standard identify the sector to be a valid bootable bootsector by checking AA55h as the last word, otherwise it should not attempt to boot off it.

I see nothng wrong with ORG 0 and doing a jmp far 7C0:start, which I find it safer then using ORG 7C00 and assuming that the BIOS has left you with CS=0. Although you must specify the code is 16bits, otherwise if the assembler thinks its 32bits it will assemble a 32bit offset of "start" offset.

Start:          
    mov ax,cs      
    mov ds,ax
    mov es,ax

Chase@OSDev wrote: The short jmp followed by the nop *precedes* the BPB(not sure if you meant layout or position when you said "follows").

Chase@OSDev wrote: Most systems will work without the jump being short and most systems require the boot signature at the end but not all.

Chase@OSDev wrote: You are right about the code segment. Lots of people make the mistake of doing something like this:

Code: Select all

Start:          
    mov ax,cs      
    mov ds,ax
    mov es,ax

Ryu wrote: Sorry, I'm actually a bad speller and in grammer.

Ryu wrote: Also means the nop is just padding so that BPB is where is suppose to be.

Ryu wrote:If you don't wish to follow FAT12 then can very well be a jmp far which will take up 5 bytes or even no jump at all.

Ryu wrote: And the other hand, something like DS=ES=7C00h and leaving CS alone, would be unsafe. Basically because you have assumed CS would be something.

To explain a bit, this goes back to differnt jmps and calls. For jumps specifically, the jump near operand is relative with CS segment, ex. jmp near 0004h which the processor will jmp to addess CS:0004h. Now, because no one ever stated any standard to set at a particular segment, CS is ambigous on differnt systems. Which means if the BIOS runs the boot sector with CS=0, your boot sector will run into problems, while a jmp far before hand to set CS would assure that its on the right track. Hope this makes any sense.

Ryu wrote:To explain a bit, this goes back to differnt jmps and calls. For jumps specifically, the jump near operand is relative with CS segment, ex. jmp near 0004h which the processor will jmp to addess CS:0004h. Now, because no one ever stated any standard to set at a particular segment, CS is ambigous on differnt systems. Which means if the BIOS runs the boot sector with CS=0, your boot sector will run into problems, while a jmp far before hand to set CS would assure that its on the right track.

blip wrote:
Near jumps work relative to IP and the encoded operand is a signed word. Doing JMP near 0004h with the instruction assembled to address 0000h will encode the jump to hop 4 bytes ahead of the first byte of the jump instruction regardless of the actual offset that the code is loaded at, the word operand will have a value of 1.

Short jumps are a shorter version with a signed byte instead, except that since this form is one byte shorter the encoded operand would have to be 2 since it is relative to the IP while the jump is being executed, which points to the byte after the jump instruction itself.

blip wrote: You weren't clear on this and I wanted to make sure you knew near jumps don't encode absolute offsets. The 3 byte size of a near jump is the same as the 3 byte area before the BPB because you may want to jump further than 127 bytes ahead in one hop, which is possible to want since the sector is 512 bytes.