Add answer of Q70

questions/answers/070.md

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+
+# Question 70
+
+
+Modify the macro the way it would produce a bit table, taking 8 times less space in memory.
+Add a function that will check number for primarity and return 0 or 1, based on this precomputed table.
+
+Hint: for the macro you will probably have to copy and paste a lot.
+
+
+# Answer
+
+```asm
+bits 64
+
+%define prime_limit 1280
+section .data
+
+; Original version.
+prime_sieve_bytes:
+    db 0, 0, 1
+%assign n 3
+%rep prime_limit
+    %assign current 1
+    %assign i 1
+        %rep n / 2
+           %assign i i + 1
+           %if (n % i) = 0
+                %assign current 0
+                %exitrep
+           %endif
+        %endrep
+db current ; n
+    %assign n n+1
+%endrep
+
+; Bit table version.
+prime_sieve_bits:
+    db 0b10101100
+%assign count_bytes prime_limit / 8
+%assign cnt 8
+%rep count_bytes
+    %assign bit 0
+    %assign mask 1
+    %rep 8 ; 8bits
+        %assign i 1
+        %assign is_p 1
+        %rep cnt / 2
+           %assign i i + 1
+           %if (cnt % i) = 0
+                %assign is_p 0
+                %exitrep
+           %endif
+        %endrep
+
+        %if is_p = 1
+            %assign bit bit | mask
+        %endif
+        %assign mask mask << 1
+
+        %assign cnt cnt + 1
+    %endrep
+    db bit
+%endrep
+
+msg_succeed:
+    db 0x0A, "Succeed !", 0x0A, 0
+
+msg_failed:
+    db 0x0A, "Failed...", 0x0A, 0
+
+
+
+section .text
+global _start
+_start:
+    push rbx
+    xor rbx, rbx
+
+.loop:
+    cmp rbx, prime_limit
+    je .break
+
+    ; Result 1
+    mov rdi, rbx
+    call is_prime_bytes
+    mov r8, rax
+
+    ; Result 2
+    mov rdi, rbx
+    call is_prime_bits
+    mov r9, rax
+
+    cmp rax, 1
+    jne .skip_print_prime
+
+    ; Print prime number.
+    mov rdi, rbx
+    call print_uint
+    mov rdi, ','
+    call print_char
+
+.skip_print_prime:
+    inc rbx
+
+    ; Compare the two results.
+    cmp r8, r9
+    je .loop
+
+    mov rdi, msg_failed
+    call print_string
+
+.break:
+    pop rbx
+
+    mov rdi, msg_succeed
+    call print_string
+
+    ; Exit
+    mov rax, 60
+    xor rdi, rdi
+    syscall
+
+
+; bool is_prime_bytes(size_t n).
+is_prime_bytes:
+    xor rax, rax
+    mov al, [prime_sieve_bytes + rdi]
+    ret
+
+
+; bool is_prime_bytes(size_t n).
+is_prime_bits:
+    mov rax, rdi
+    shr rax, 3
+
+    mov al, [prime_sieve_bits + rax]
+
+    mov rcx, rdi
+    and rcx, 0b111
+    shr al, cl
+    and rax, 0x1
+    ret
+
+
+string_length:
+    mov rax, rdi
+
+.loop:
+    cmp byte [rax], 0
+    je .end
+
+    inc rax
+    jmp .loop
+
+.end:
+    sub rax, rdi
+    ret
+
+
+print_string:
+    mov rsi, rdi
+    call string_length
+
+    mov rdi, 1
+    mov rdx, rax
+    mov rax, 1
+    syscall
+
+    ret
+
+
+print_newline:
+    mov rdi, 0x0A
+
+
+print_char:
+    push rdi
+
+    mov rax, 1
+    mov rdi, 1
+    mov rsi, rsp
+    mov rdx, 1
+    syscall
+
+    pop rdi
+
+    xor rax, rax
+    ret
+
+
+print_uint:
+    mov rax, rdi
+    mov rcx, 10
+
+    ; Push null character.
+    push 0
+    add rsp, 7
+    mov r10, 1
+
+.loop:
+    xor rdx, rdx
+    inc r10
+
+    div rcx
+    add rdx, 0x30
+
+    ; Cannot push 8bit into stack directory.
+    shl rdx, 8
+    push dx
+    inc rsp
+
+    test rax, rax
+    jnz .loop
+
+    mov rdi, rsp
+    call print_string
+
+    add rsp, r10
+
+    xor rax, rax
+    ret
+```