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# Count the characters in a string (until a null byte is reached)
# It's supposed to behave similarly with strlen()
#
# Returns the count in %rax
#
# - %rcx: Char count
# - %rdx: current Char address
# - %al: current char
#
.type count_chars, @function
.globl count_chars
# We receive the string addr in the stack.
# Remember %rsp + 8 contains the return value
.equ ST_STRING_ADDRESS, 16
count_chars:
pushq %rbp
movq %rsp, %rbp
#initialize counter
movq $0, %rcx
# Start address of string:
movq ST_STRING_ADDRESS(%rbp), %rdx
count_loop_begin:
# Grab char
movb (%rdx), %al
cmpb $0, %al
je count_loop_end
# We are not done yet...
incq %rcx
incq %rdx
jmp count_loop_begin
count_loop_end:
movq %rcx, %rax
movq %rbp, %rsp
popq %rbp
ret
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