2 files changed, 36 insertions, 0 deletions
diff --git a/zap/source/amd64/mem/strlen.S b/zap/source/amd64/mem/strlen.S
index bd83008..e8739e2 100644
--- a/zap/source/amd64/mem/strlen.S
+++ b/zap/source/amd64/mem/strlen.S
@@ -6,6 +6,7 @@
 
 zap_strlen:
 # rax: Address of the current character.
+ # rdi: Address of the first character.
 # rdx: Current character.
 
 movq %rdi,%rax
diff --git a/zap/source/amd64/mem/utf20len.S b/zap/source/amd64/mem/utf20len.S
new file mode 100644
index 0000000..5af352b
--- /dev/null
+++ b/zap/source/amd64/mem/utf20len.S
@@ -0,0 +1,35 @@
+# 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:
+ # rax: Address of the current character.
+ # rdi: Address of the first character.
+ # rdx: Current character.
+
+ movq %rdi,%rax
+
+ # Iterate over the string:
+.loop:
+
+ # Move the character into a register:
+ movl (%rax),%edx
+
+ # Check if we have reached the null-terminator:
+ testl %edx,%edx
+ jz .done # If so, we are done.
+
+ # Continue to the next character:
+ addq $0x4,%rax
+ jmp .loop
+
+ # Done:
+.done:
+
+ # Get the length:
+ subq %rdi,%rax
+ shrq $0x2,%rax # Divide by four to get the number of doublewords rather than bytes.
+
+ ret