add counter2 program that is more efficient

counter/compile

@@ -1,3 +1,4 @@
- nasm -f elf64 counter.asm -o counter.o
+ #nasm -f elf64 counter.asm -o counter.o
+ nasm -f elf64 -g -F dwarf counter.asm -o counter.o
  ld counter.o -o counter
  ./counter

counter2/compile

@@ -0,0 +1,4 @@
+ #nasm -f elf64 counter.asm -o counter.o
+ nasm -f elf64 -g -F dwarf counter.asm -o counter.o
+ ld counter.o -o counter
+ ./counter

counter2/counter.asm

@@ -0,0 +1,100 @@
+; nasm -f elf64 count.asm -o count.o
+; ld count.o -o count
+; ./count
+
+BITS 64
+SECTION .data
+    prefix db "Line ", 0         ; constant prefix string
+    newline db 10, 0             ; newline character "\n"
+
+SECTION .bss
+    numbuf resb 16               ; buffer to hold number string
+
+SECTION .text
+    global _start
+
+;----------------------------------------
+; _start: Entry point
+;----------------------------------------
+_start:
+    mov rcx, 100              ; use rcx / loop as an easy counter.
+
+print_loop:
+    ; Print "Line "
+    mov rsi, prefix
+    call print_str
+
+    ; Convert counter to string
+    mov rax, 101             ; as the rcx couns backwards we have to subtract it from 100 to have ascending order
+    sub rax, rcx
+    mov rdi, numbuf
+    call int_to_str
+
+    ; Print number
+    mov rsi, numbuf
+    call print_str
+
+    ; Print newline
+    mov rsi, newline
+    call print_str
+
+    loop print_loop          ; will automatically decrement rcx and stop looping if it's 0
+
+    ; Exit syscall
+    mov rax, 60
+    xor rdi, rdi
+    syscall
+
+;----------------------------------------
+; print_str: rsi = null-terminated string
+;----------------------------------------
+print_str:
+    mov r8, rcx
+    push rsi
+    mov rdi, rsi
+    xor rcx, rcx
+.find_len:
+    cmp byte [rdi+rcx], 0
+    je .got_len
+    inc rcx
+    jmp .find_len
+.got_len:
+    mov rax, 1          ; sys_write
+    mov rdi, 1          ; fd = stdout
+    mov rdx, rcx        ; length
+    mov rsi, rsi        ; buffer
+    syscall
+    pop rsi
+    mov rcx, r8
+    ret
+
+;----------------------------------------
+; int_to_str: rax = number, rdi = buffer
+; Converts integer in RAX to ASCII string
+; (null-terminated, left-justified)
+;----------------------------------------
+int_to_str: 
+    mov r8, rcx
+    mov rcx, 0
+    mov rbx, 10
+
+.int_div_loop:
+    xor rdx, rdx
+    div rbx             ; quotient in rax, remainder in rdx
+    push rdx
+    inc rcx
+    test rax, rax
+    jnz .int_div_loop
+
+    ; Write digits to buffer
+    mov rsi, rdi
+.write_digits:
+    pop rdx
+    add dl, '0'
+    mov [rsi], dl
+    inc rsi
+    loop .write_digits
+
+    mov byte [rsi], 0   ; null terminator
+    mov rcx, r8
+    ret