如何在NASM汇编程序中打印以下内容？

Question

I am still learning how to use NASM so there may be some syntax mistakes in my question. Anyway, when I create a label, suppose

buf: resb 16

Then, the label buf contains a memory address that points to the first of the 16 bytes that were reserved. Then, I want to print everything from an arbitrary byte in those 16 to the very last one, say, print the bytes 13 through 16. Then the code would look something like

mov eax, 4
mov ebx, 1
mov ecx, buf + 12
mov edx, 4         ;; for the 4 bytes to be read 
int 80h

Is this correct? Then, what I am actually trying to do is

some_number:     resb 1

mov [some_number], byte 3

mov eax, 4
mov ebx, 1
mov ecx, buf + 16 - [some_number]
mov edx, byte [some_number]         ;; for the 3 bytes to be read, in this case 
int 80h

so I want to pass the address buf + (16 - 3) which is buf + 13. But this definitely does not work. Let me know if this is wrong or if there is a better way to achieve this.

By the way, some_number will be determined by an arbitrary process so it will be different each time my program runs.

Answer 1

mov ecx, buf + 12 is invalid, but you can achieve that by using lea ecx, [buf + 12] which is pretty much " load effective address of buf + 12 " and that's what you want.

While the effective address format does allow registers, it doesn't allow indirection or subtraction, so lea ecx, [buf + 16 - [some_number]] would be doubly invalid. You will have to do the subtraction in a separate step, taking care to use the proper operand size. One option would be:

lea ecx, [buf + 16]
movzx eax, byte [some_number]
sub ecx, eax

If your some_number were a dword not a byte, you could simplify that to:

lea ecx, [buf + 16]
sub ecx, [some_number]