Added a basic calculator in assembly x86
This commit is contained in:
Fabio Scotto di Santolo
233
calc.asm
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233
calc.asm
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; A simple calculator program that do simple operation (+, -, *, /)
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;
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; Author: Fabio Scotto di Santolo
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; Date: 03/07/2025
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global _start
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section .text
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_start:
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; Prompt for first number:
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mov eax, 0x04 ; write syscall
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mov ebx, 1 ; STDOUT file descriptor
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mov ecx, nprompt
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mov edx, len1
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int 0x80 ; trigger write syscall
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mov eax, 0x03 ; read syscall
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mov ebx, 0 ; STDIN file descriptor
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mov ecx, buffer1
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mov edx, 32
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int 0x80 ; trigger read syscall
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mov edi, buffer1 ; save buffer address
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mov esi, eax ; save length
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; Prompt for second number:
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mov eax, 0x04 ; write syscall
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mov ebx, 1 ; STDOUT file descriptor
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mov ecx, nprompt
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mov edx, len1
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int 0x80 ; trigger write syscall
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mov eax, 0x03 ; read syscall
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mov ebx, 0 ; STDIN file descriptor
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mov ecx, buffer2
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mov edx, 32
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int 0x80 ; trigger read syscall
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; Convert second number
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mov ecx, buffer2 ; save buffer address
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mov ebx, eax ; save length
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call ascii_to_int
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push eax ; save converted second number in the stack
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; Convert first number
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mov ecx, edi ; ECX = address
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mov ebx, esi ; EBX = length
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call ascii_to_int ; EAX = first number
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push eax ; save converted first number in the stack
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; Prompt for operand:
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mov eax, 0x04 ; write syscall
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mov ebx, 1 ; STDOUT file descriptor
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mov ecx, oprompt
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mov edx, len2
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int 0x80 ; trigger write syscall
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mov eax, 0x03 ; read syscall
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mov ebx, 0 ; STDIN file descriptor
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mov ecx, buffer3
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mov edx, 1
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int 0x80 ; trigger read syscall
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; Check operand
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mov eax, [buffer3] ; move in EAX operand
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cmp eax, 0x2B ; compare with +
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je .add
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cmp eax, 0x2D ; compare with -
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je .sub
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cmp eax, 0x2A ; compare with *
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je .mul
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cmp eax, 0x2F ; compare with /
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je .div
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jmp end ; safety jmp to end
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.add:
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pop eax
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pop ebx
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add eax, ebx
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jmp result
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.sub:
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pop eax
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pop ebx
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sub eax, ebx
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jmp result
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.mul:
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pop eax
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pop ebx
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imul eax, ebx
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jmp result
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.div:
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pop eax
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pop ebx
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xor edx, edx
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idiv ebx
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jmp result
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result:
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; Print operation result
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mov ebx, eax
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mov eax, msg
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call concat_string_num ; concat result with final message
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mov eax, 0x04 ; write syscall
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mov ebx, 1 ; STDOUT file descriptor
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int 0x80 ; trigger syscall
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jmp end
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end:
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; call syscall exit
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mov eax, 0x1
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xor ebx, ebx ; reset register to 0
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int 0x80 ; trigger syscall
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; --- ASCII to Integer Conversion Routine ---
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ascii_to_int:
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push ebx
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push ecx
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push edx
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push esi
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xor eax, eax
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xor edx, edx
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mov esi, ecx
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mov ecx, ebx
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convert_loop:
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cmp byte [esi], 0xA ; newline?
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je convert_done
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cmp byte [esi], 0xD ; carriage return?
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je convert_done
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mov ebx, 10
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mul ebx ; EAX *= 10
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movzx ebx, byte [esi]
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sub ebx, '0'
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add eax, ebx ; EAX += digit
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inc esi
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loop convert_loop
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convert_done:
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pop esi
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pop edx
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pop ecx
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pop ebx
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ret
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; ----------------------------------------------------------------
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; concat_string_num
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; Concatenates a null-terminated string in EAX with a number in EBX.
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; Result:
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; ECX = pointer to result string (concat_buf)
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; EDX = length of result string (excluding null terminator)
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; Clobbers:
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; EAX, EBX, ESI, EDI, EDX, AL, ECX
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; ----------------------------------------------------------------
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concat_string_num:
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push eax
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push ebx
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push esi
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push edi
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; Copy string from [EAX] to concat_buf
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mov esi, eax
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mov edi, concat_buf
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.copy_str:
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lodsb
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stosb
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test al, al
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jnz .copy_str
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dec edi ; go back to overwrite null terminator
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; Convert EBX (number) to ASCII in num_buf (reversed order)
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mov eax, ebx ; number to convert
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mov esi, num_buf + 11 ; write from the end
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mov byte [esi], 0 ; null terminator
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.convert_digit:
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dec esi
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xor edx, edx
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mov ebx, 10
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div ebx ; EAX = EAX / 10, EDX = remainder
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add dl, '0'
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mov [esi], dl
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test eax, eax
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jnz .convert_digit
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; Copy number string from [ESI] to buffer (at current EDI)
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mov ecx, num_buf + 12
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sub ecx, esi ; ECX = number string length
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rep movsb
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; Add newline at end
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mov byte [edi], 0xA
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inc edi
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; Null terminator
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mov byte [edi], 0
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; Output: ECX = buffer, EDX = length
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mov ecx, concat_buf
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mov edi, concat_buf
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xor edx, edx
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.calc_len:
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cmp byte [edi + edx], 0
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je .done
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inc edx
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jmp .calc_len
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.done:
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pop edi
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pop esi
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pop ebx
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pop eax
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ret
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section .data
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nprompt: db "insert a number: "
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len1: equ $ - nprompt
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oprompt: db "insert a operand (+, -, * or /): "
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len2: equ $ - oprompt
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msg: db "Result is ", 0
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msglen: equ $ - msg
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section .bss
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buffer1: resb 32
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buffer2: resb 32
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buffer3: resb 1
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concat_buf: resb 256
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num_buf: resb 12
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