From a7253bacdedf2fb1b72d8f760e1f469e6c0b96bf Mon Sep 17 00:00:00 2001
From: Tijmen van Nesselrooij <tijmen_222@hotmail.com>
Date: Tue, 26 Mar 2019 19:52:31 +0100
Subject: [PATCH] rot13 assignment added

---
 ...{3_homework.asm => 3_homework_highest.asm} |  0
 x86_64/3_homework_rot13.asm                   | 79 +++++++++++++++++++
 2 files changed, 79 insertions(+)
 rename x86_64/{3_homework.asm => 3_homework_highest.asm} (100%)
 create mode 100644 x86_64/3_homework_rot13.asm

diff --git a/x86_64/3_homework.asm b/x86_64/3_homework_highest.asm
similarity index 100%
rename from x86_64/3_homework.asm
rename to x86_64/3_homework_highest.asm
diff --git a/x86_64/3_homework_rot13.asm b/x86_64/3_homework_rot13.asm
new file mode 100644
index 0000000..cd83be7
--- /dev/null
+++ b/x86_64/3_homework_rot13.asm
@@ -0,0 +1,79 @@
+global _start
+
+section .data
+  str: db "Please give me a string: " ; \n = 0xA
+  strSize: equ $ - str
+
+  stdin: equ 0  ; POSIX
+  stdout: equ 1 ; POSIX
+
+section .bss
+  strBuffer: resb 64    ; reserve 64 bytes
+  strBufferSize: equ 64 ; same as above
+
+section .text
+_start:
+  mov rax, 1 ; Print str to screen
+  mov rdi, stdout
+  mov rsi, str
+  mov rdx, strSize
+  syscall
+
+  xor rax, rax ; Read user input
+  mov rdi, stdin
+  mov rsi, strBuffer
+  mov rdx, strBufferSize
+  syscall
+
+  mov rbx, rax ; store bytes read
+  dec rbx ; subtract the newline
+  xor rcx, rcx ; make register C our loop var
+start_loop:
+  cmp rcx, rbx 
+  jge print
+
+  ; Calculate the position
+  mov rdx, strBuffer
+  add rdx, rcx
+  ; Load the char at the position
+  xor rax, rax
+  mov al, byte [rdx]
+  mov r8, rdx
+  ; Check if we're upper or lower case
+  mov rdx, 95
+  cmp rax, rdx
+  jge lower_case
+upper_case:
+  mov r9, 91 ; 90 is the highest valid upper case value
+  jmp algorithm
+lower_case:
+  mov r9, 123 ; 122 is the highest lower case value
+algorithm:
+  ; Shift it 13 positions
+  mov rdx, 13
+  add rax, rdx
+  ; Check if we should wrap around
+  cmp rax, r9
+  jl loop_end
+  jmp ascii_wrap
+ascii_wrap:
+  ; Wrap around
+  mov rdx, 26
+  sub rax, rdx
+loop_end:
+  ; Store the result
+  mov byte [r8], al
+  inc rcx
+  jmp start_loop
+
+print:
+  inc rbx ; Add the newline back to it
+  mov rax, 1 ; Print str to screen
+  mov rdi, stdout
+  mov rsi, strBuffer
+  mov rdx, rbx
+  syscall
+
+  mov rax, 60
+  xor rdi, rdi
+  syscall