FEAT Added subroutines to the list of skills taught in this tutorial.

.gitignore

@@ -6,3 +6,5 @@ hello32
 hello64
 counted-hello32
 counted-hello64
+subroutine-hello32
+subroutine-hello64

Makefile

@@ -23,5 +23,13 @@ counted-hello64: counted-hello64.s    ## Build the 32 bit version of Project 3
 	nasm -f elf64 counted-hello64.s
 	ld -o counted-hello64 counted-hello64.o
 
+subroutine-hello32: subroutine-hello32.s    ## Build the 32 bit version of Project 3
+	nasm -f elf subroutine-hello32.s
+	ld -m elf_i386 -o subroutine-hello32 subroutine-hello32.o
+
+subroutine-hello64: subroutine-hello64.s    ## Build the 32 bit version of Project 3
+	nasm -f elf64 subroutine-hello64.s
+	ld -o subroutine-hello64 subroutine-hello64.o
+
 clean:                ## Delete all built and intermediate features
 	rm -f hello32 hello64 counted-hello32 counted-hello64 *.o

subroutine-hello32.s

@@ -0,0 +1,66 @@
+    ;; Hello World Program #1
+    ;; Compile with: nasm -f elf hello.s
+    ;; Link with: ld -m elf_i386 -o hello hello.o
+    ;; Run with: ./hello
+
+    ;; sys/unistd_32.h
+%define     SYS_write	4
+%define     SYS_exit	1
+
+    ;; unistd.h
+%define STDOUT		1
+
+section .data
+    msg     db  "Okay, so we're doing 32-bit subroutines now, huh?", 0Ah, 00h
+
+section .text
+    global _start
+
+_start:
+    mov     eax, msg            ; Put the address of our message into eax.
+    call    strlen              ; call the function strlen.  We're using EAX as our argument.
+
+    mov     edx, eax            ; We know printit wants these two
+    mov     eax, msg
+    call    printit
+    call    exit
+
+strlen:
+    push    ebx                 ; Push ebx onto the stack, since the calling scope
+                                ; will probably want its state restored correctly, right?
+    mov     ebx, eax
+
+strlen_next:
+    cmp     byte [eax], 0
+    jz      strlen_done
+    inc     eax
+    jmp     strlen_next
+
+strlen_done:
+    sub     eax, ebx            ; Straight from the counted-hello file
+    pop     ebx                 ; restore EBX for the calling scope, which is
+                                ; expecting its answer in eax
+    ret
+
+    ;; Takes EAX as the address of the message and EDX as the
+    ;; length, and prints them.  Restores all used registers
+    ;; when finished.
+printit:
+    push    eax
+    push    ebx
+    push    ecx
+    mov     ecx, eax
+    mov     ebx, STDOUT
+    mov     eax, SYS_write
+    int     80h
+    pop     ecx
+    pop     ebx
+    pop     eax
+    ret
+
+    ;; Since this terminates the program, I'm not worried about
+    ;; managing the stack correctly.
+exit:
+    mov     ebx, 0
+    mov     eax, SYS_exit
+    int     80h

subroutine-hello64.s

@@ -0,0 +1,66 @@
+    ;; Hello World Program #1
+    ;; Compile with: nasm -f elf hello.s
+    ;; Link with: ld -m elf_i386 -o hello hello.o
+    ;; Run with: ./hello
+
+    ;; sys/unistd_32.h
+%define     SYS_write	1
+%define     SYS_exit	60
+
+    ;; unistd.h
+%define STDOUT		1
+
+section .data
+    msg     db  "Okay, so we're doing 64-bit subroutines now, huh?", 0Ah, 00h
+
+section .text
+    global _start
+
+_start:
+    mov     rsi, msg            ; Put the address of our message into rax.
+    call    strlen              ; call the function strlen.  We're using RAX as our argument.
+    ;; At this point, RDX is length and RSI still points to the source.
+    ;; The differences here make me wonder if my understanding of the 32-bit
+    ;; version are incorrect, since now the two fields that matter are
+    ;; already populated.
+    ;; 
+    ;; Note that this is non-standard.  RAX is usually assumed to hold
+    ;; the return value, but I'm cheating by knowing that printit
+    ;; wants the result of strlen in RDX.  Whether this is exploitable
+    ;; in a compiler (you know, that whole "choosing registers"
+    ;; question) is something to ponder.
+    
+    call    printit
+    call    exit
+
+strlen:
+    mov     rdx, rsi
+
+strlen_next:
+    cmp     byte [rdx], 0
+    jz      strlen_done
+    inc     rdx
+    jmp     strlen_next
+
+strlen_done:
+    sub     rdx, rsi
+    ret
+
+    ;; Takes RDI as the address of the message and RDX as the length,
+    ;; and prints them.  Restores all used registers when finished.
+printit:
+    push    rdi
+    push    rax
+    mov     rdi, STDOUT         ; using STDOUT (see definition above)
+    mov     rax, SYS_write      ; Using WRITE in 32-bit mode?
+    syscall
+    pop     rax
+    pop     rdx
+    pop     rdi
+
+    ;; Since this terminates the program, I'm not worried about
+    ;; managing the stack correctly.
+exit:
+    mov     rdi, 0
+    mov     rax, SYS_exit
+    syscall