factorial

.gitignore

@@ -0,0 +1 @@
+*.o

Dockerfile

@@ -1,11 +0,0 @@
-FROM ubuntu:latest
-
-RUN apt-get update && apt-get install -y nasm gcc gdb vim
-
-COPY main.asm /app/main.asm
-
-WORKDIR /app
-RUN nasm -f elf64 main.asm -o main.o
-RUN ld main.o -o main
-
-CMD ["./main"]

Makefile

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+ASM      = as  # GNU Assembler
+LD       = gcc # GCC Linker
+LD_FLAGS = -nostartfiles
+
+SRC = factorial.s
+OBJ = factorial.o
+OUT = factorial
+
+all: $(OUT)
+
+$(OBJ): $(SRC)
+	$(ASM) $(SRC) -o $(OBJ)
+
+$(OUT): $(OBJ)
+	$(LD) $(LD_FLAGS) $(OBJ) -o $(OUT)
+
+run: all
+	./$(OUT)
+
+clean:
+	rm -f $(OBJ) $(OUT)

README.md

@@ -1,15 +1,3 @@
 # Assembly Project Demonstration - Factorial with Recursion
 Demonstrate how functoin calls work in assembly by using
 a factorial recursion example.
-
-## Build
-
-```bash
-docker build -t asm .
-```
-
-## Run
-
-```bash
-docker run asm
-```

factorial.c

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+#include <stdio.h>
+
+int factorial(int n)
+{
+    if (n <= 1)
+    {
+        return n;
+    }
+
+    return n * factorial(n - 1);
+}
+
+int main(void)
+{
+    int f;
+
+    f = factorial(5);
+
+    printf("%d\n", f);
+
+    return 0;
+}

factorial.s

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+.intel_syntax noprefix # allow mov rax, 1 syntax in GAS (GNU Assembly)
+.global _start
+
+# factorial
+# - rdi as input number
+# - rax as result
+factorial:
+    push rbp
+    mov rbp, rsp
+
+    cmp rdi, 1
+    jle base_case      # if n <= 1 return 1
+
+    push rdi           # save n
+    dec rdi            # rdi = n-1
+    call factorial     # rax = factorial(n-1)
+
+    pop rdx            # rdx = original n
+    imul rax, rdx      # rax = factorial(n-1) * n
+    jmp done
+
+base_case:
+    mov rax, 1
+
+done:
+    mov rsp, rbp
+    pop rbp
+    ret
+
+print_rax_digit:
+    sub rsp, 16
+
+    mov rbx, 10
+
+    # ones digit
+    xor rdx, rdx
+    mov rcx, rax
+    div rbx
+    add dl, '0'
+    mov [rsp+2], dl
+
+    # tens digit
+    xor rdx, rdx
+    mov rcx, rax
+    div rbx
+    add dl, '0'
+    mov [rsp+1], dl
+
+    # hundreds digit
+    add al, '0'
+    mov [rsp], al
+
+    mov rax, 1
+    mov rdi, 1
+    mov rsi, rsp
+    mov rdx, 3
+    syscall
+
+    add rsp, 16
+    ret
+
+_start:
+    mov rdi, 5 # Use 5 for our example input
+    mov rax, 1 # Set starting result to 1
+
+    call factorial
+
+    call print_rax_digit
+
+    mov rax, 60 # syscall exit - 64 bit
+    mov rdi, 0  # exit code
+    syscall