From 26379ad01fab04b78200c8e3d976397c8b69c53d Mon Sep 17 00:00:00 2001
From: Tijmen van Nesselrooij <tijmen_222@hotmail.com>
Date: Tue, 13 Aug 2019 18:59:36 +0200
Subject: [PATCH] non leaf function tutorial added

---
 x86_64/5_nonleaf.asm | 163 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 163 insertions(+)
 create mode 100644 x86_64/5_nonleaf.asm

diff --git a/x86_64/5_nonleaf.asm b/x86_64/5_nonleaf.asm
new file mode 100644
index 0000000..c61bee0
--- /dev/null
+++ b/x86_64/5_nonleaf.asm
@@ -0,0 +1,163 @@
+global _start
+
+section .bss
+	BUF_SIZE: equ 64
+	buf1: resb 64
+	buf2: resb 64
+
+section .text
+_start:
+	; Calling convention of a non leaf function is the same
+	; as a leaf function
+	mov rdi, 40579
+	mov rsi, buf1
+	mov rdx, BUF_SIZE
+	call uint2str
+
+	; write result to stdout
+	mov rdx, rax
+	mov rax, 1
+	mov rdi, 1
+	mov rsi, buf1
+	syscall
+
+	; Calculate !5
+	mov rdi, 5
+	call factorial
+
+	; Convert result to string
+	mov rdi, rax
+	mov rsi, buf2
+	mov rdx, BUF_SIZE
+	call uint2str
+
+	; Write !5 to stdout
+	mov rdx, rax
+	mov rax, 1
+	mov rdi, 1
+	mov rsi, buf2
+	syscall
+
+	; Exit
+	mov rax, 60
+	xor rdi, rdi
+	syscall
+
+; size_t uint2str(unsigned number, char * buffer, size_t bufferSize)
+; Returns the number of bytes written to the buffer
+uint2str:
+	push rbp ; Store the rbp register on the stack
+	mov rbp, rsp ; Overwrite rbp with the function local stack pointer
+	; Backup the remaining callee saved registers
+	push rbx
+	push r12
+	push r13
+	push r14
+	push r15
+
+	; Overwrite the saved registers for demonstration purposes
+	mov r12, rdi ; number
+	mov r13, rsi ; buffer
+	mov r14, rdx ; bufferSize
+
+	xor r15, r15 ; counter
+	mov rax, r12 ; working copy of number
+
+.loop:
+	cmp rax, 0
+	je .done ; n == 0
+	cmp r15, r14
+	jge .done ; i > BUF_SIZE
+
+	; n % 10
+	xor rdx, rdx
+	mov rdi, 10
+	div rdi ; rax = rax / rdi
+
+	add dl, '0' ; rdx = rax % rdi
+	mov byte [r13 + r15], dl
+
+	inc r15
+	jmp .loop
+.done:
+	mov byte [r13 + r15], 0x0a ; append newline
+	inc r15
+	
+	; Call reverse string routine to fix the order
+	mov rdi, r13 ; buffer
+	lea rsi, [r15 - 1] ; number of characters written, excluding newline
+	call revstr
+
+	mov rax, r15 
+
+	; Restore saved registers
+	pop r15
+	pop r14
+	pop r13
+	pop r12
+	pop rbx
+
+	; restore rsp from rbp
+	mov rsp, rbp
+	pop rbp ; restore rbp
+
+	ret
+
+; unsigned factorial(unsigned n)
+; recursive!
+factorial:
+	; save registers
+	push rbp
+	mov rbp, rsp
+	push rbx
+	push r12
+	push r13
+	push r14
+	push r15
+
+	mov r12, rdi; n
+
+	cmp r12, 0
+	je .zero
+
+	mov rbx, rdi
+	dec rdi
+	call factorial ; recursion!
+
+	xor rdx, rdx
+	mul rbx
+
+	jmp .done
+.zero:
+	mov rax, 1 ; !0 = 1
+.done:
+	; Restore saved registers
+	pop r15
+	pop r14
+	pop r13
+	pop r12
+	pop rbx
+
+	; restore rsp from rbp
+	mov rsp, rbp
+	pop rbp ; restore rbp
+	ret
+
+; void revstr(char * s, size_t size)
+revstr:
+	xor r8, r8 ; lowerbound
+	lea r9, [rsi - 1] ; upperbound
+
+.loop:
+	cmp r8, r9
+	jge .done
+
+	mov cl, byte [rdi + r8] ; tmp = s[lowerbound]
+	xchg cl, byte [rdi + r9] ; tmp <-> s[upperbound]
+	mov byte [rdi + r8], cl ;  s[lowerbound] = tmp
+
+	inc r8
+	dec r9
+	jmp .loop
+.done:
+	ret