gistfile1.asm

[bits 64]

extern rand
extern time
extern srand
extern write
global main

section .text
%define SIZE (31*2048)

main:
    ; Initialize srand
    xor rdi, rdi
    call time
    mov rdi, rax
    call srand

    ; rbp will always point to the buffer
    mov rbp, buf

loop_outer:
    ; Zero out rbx
    xor ebx, ebx

loop_inner:
    ; Get a random 31-bit number into rax
    call rand

    ; Put the random bits into xmm1 and xmm2
    movd xmm1, eax
    movd xmm2, eax

    ; Put the lower 16 bits in xmm1, and the upper 15 in xmm2
    pshufb xmm1, [shuffler1]
    pshufb xmm2, [shuffler2]

    ; Zero out xmm0 
    pxor xmm0, xmm0

    ; Put a single bit from the random number into each byte of xmm0/xmm1
    pand xmm1, [and1]
    pand xmm2, [and1]

    ; Set each byte to 0xff if the bit was 0, otherwise 0
    pcmpeqb xmm1, xmm0
    pcmpeqb xmm2, xmm0

    ; Mask out the difference between '/' and '\'
    pand xmm1, [and2]
    pand xmm2, [and2]

    ; Add the value for '/', such that each byte is now either '/ or '\'
    paddb xmm1, [adder]
    paddb xmm2, [adder]

    ; Save the 31 bytes
    movdqu [rbp + rbx], xmm1
    movdqu [rbp + rbx + 16], xmm2
    add ebx, 31

    ; Check if done
    cmp ebx, SIZE
    jne loop_inner

    ; Print them
    mov rdi, 1
    mov rsi, rbp
    mov rdx, SIZE
    call write

    ; Go again
    jmp loop_outer


section .data
    align 16
    shuffler1:  db 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1
    shuffler2:  db 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3
    and1:       db 0x1, 0x2, 0x4, 0x8, 0x10, 0x20, 0x40, 0x80, 0x1, 0x2, 0x4, 0x8, 0x10, 0x20, 0x40, 0x80
    and2:       times 16 db 0x2d
    adder:      times 16 db 0x2f

section .bss
    buf: resb SIZE