.X64
START SEGMENT USE64 para public 'text'
PUBLIC _start

ASSUME cs:START
ASSUME ds:BSS

;----------------------------------------;
; Program to copy two arbitrary          ;
; buffers                                ;
;----------------------------------------;
_start PROC NEAR                         ;
    ;-- get current program brk          ;
    mov rax, 12                          ; SYS_BRK
    mov rdi, 0                           ; invalid adress
    syscall                              ;
    mov QWORD PTR [OLD_PROGRAM_BRK], rax ; store the old program break
    ;-- Allocate memory -----------------;
    mov rdi, rax                         ; rbx = current break
    mov rax, 12                          ; SYS_BRK
    add rdi, 20000                       ; reserve 20000 bytes
    syscall                              ;
    mov QWORD PTR [PROGRAM_BRK], rax     ; store the new program break
;////////////////////////////////////////;
    ;-- setup a loop --------------------;
    mov rdx, 100000000                   ; 100000000 iterations
@@LOOP:                                  ;
    ;-- copy the memory -----------------;
    pop rax                              ; get buffer pointer
    mov rdi, QWORD PTR [OLD_PROGRAM_BRK] ; set source
    mov rsi, rdi                         ;
    add rsi, 10000                       ; set destination
    push rax                             ; preserve rax for next loop
    mov rax, 10000                       ; amount to copy
    call _COPY                           ; copy memory
    ;-- loop head -----------------------;
    dec rdx                              ;
    and rdx, rdx                         ; is edx zero?
    jnz @@LOOP                           ; if not, loop
;////////////////////////////////////////;
    ;-- exit ----------------------------; when done looping, stop
    mov rax, 1                           ;
    int 80h                              ;
_start ENDP                              ;
;========================================;

;----------------------------------------;
; MEMCPY                                 ;
;-input----------------------------------;
; esi -> source buf                      ;
; esd -> dest buf                        ;
; eax -> size                            ;
;----------------------------------------;
_COPY PROC NEAR                          ;
    mov rcx, rax                         ;
    and rax, 03h                         ; rax %= 4; find out how many bytes are left after copying 8packs
    sub rcx, rax                         ; only iterate as many times as possible
    shr rcx, 2                           ; divide by 4 since we are copying 4 bytes at once
    rep movsd                            ; copy as many bytes as possible in packs of 4
    mov rcx, rax                         ; copy the rest
    rep movsb                            ;
    ret                                  ;
_COPY ENDP                               ;
;========================================;
START ENDS

BSS SEGMENT '_bss'
    PROGRAM_BRK DQ ?
    OLD_PROGRAM_BRK DQ ?
BSS ENDS
END