path: root/zap/source/amd64/mem/utf20len.S

# Copyright 2022 Gabriel Jensen.
# This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
# If a copy of the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.

.globl zap_utf20len

zap_utf20len:
# Address of the current character:
#define addr  %rax
# Address of the first character:
#define start %rdi
# Current character:
#define chr   %edx

	movq start,addr

	# Iterate over the string:
.loop:

	# Move the character into a register:
	movl (addr),chr # chr = *addr

	# Check if we have reached the null-terminator:
	testl chr,chr # if (chr == 0x0)
	jz .done      # goto done

	# Continue to the next character:
	addq $0x4,addr # addr += 0x4
	jmp .loop      # goto loop

	# Done:
.done:

	# Get the length:
	subq start,addr # addr -= start
	shrq $0x2,addr  # addr /= 0x4     // Divide by four to get the number of doublewords rather than bytes.

	ret # return addr