nasm.txt

section .data
     ; eg: myName db "Shreyas",10,0; 0 is null, 10 is \n

section .text
    global _start

start:
    ;
    ;
    ;
    ;

-------------------------------------------------------
---------------------------------------------------------

SysCalls: ID,1,2,3,4,5,6 -> rax,rdi,rsi,rdx,r10,r8,r9
    1. sys_read -> 0, #filedescriptor, $buffer, #count,-,-,-
                        

    2. sys_write-> 1, #filedescriptor, $buffer,         #count,-,-,-
                      0     1       2   loc of string  no.of bytes
                      i/p  o/p    err   eg: myName


3.sys_exit
mov rax, 60  |
mov rdi, 0   |--> to exit from program
syscall      |

----------------------------------------
--------------------------------------------------------------------------------------------
REGISTERS:
rip (eip, ip)64bit reg-index pointer //holds addr of next instruction; inc by 1 to move to next 
rsp(esp, sp) - stack pointer
rbp - base pointer

r0 = rax,r1 = rcx ,r2 = rdx, r3 = rbx, r4 = rsp, r5 = rbp, r6 = rsi, r7 = rdi,
r8,r9,r10,r11,r12,r13,r14,r15
r0d = eax (lower 32 bits)
r0w = ax (lower 16 bits)
r0b = al (lower 8 bits)
ah,ch,dh,bh -> bits 15 through 8 of r0,r1,r2,r3

------------------------------------------------
---------------------------------------------------
INSTRUCTIONS:
cmp rax, rbx ; compare
je, jne, jg, jge, jl, jle, jz, jnz, jo(overflow), jno, js(signed), jns
call labelName; function call
...
labelName : ;
            ;
            ;
            ret

mul reg; rax = rax * reg
imul reg ; signed mul
div reg; rax = rax/reg
neg reg; reg = -reg
inc reg;
dec reg;
adc a,b; a = a+b+CF
sbc a,b; a = a-b-CF

------------------------------------------------
-------------------------------------------------
STACK:

push reg/value; push
pop reg; pop 
pop [addr]; pop to memory loc addr
mov reg, [rsp]; peek top of stack and store it in reg

-----------------------------------------------------
----------------------------------------------------------------
MACROS:
%macro addDigits 2; 2 is argc (no. of arguments)
        mov rax, %1;    %1 is the first parameter
        mov rbx, %2;    %2 is the second parameter
        add rax,rbx;

STDIN equ 0;    sim to #define
STDOUT equ 1
STDERR equ 2
SYS_READ equ 0
SYS_WRITE equ 1
SYS_EXIT equ 60