Lab 09 fix

labs/lab-09/reading/calling-convention.md

@@ -49,10 +49,10 @@ main:
 
     mov rdi, fmt
     mov rsi, text
+    xor rax, rax
     call printf
 
     pop rbp
-    xor rax, rax
     ret
 ```
 

labs/lab-09/reading/memory-layout-c-asm.md

@@ -40,14 +40,20 @@ section .data
 text db "291 is the best!", 10, 0
 strformat db "%s", 0
 
-section .code
+section .text
+global main
 
 main:
-        push dword text
-        push dword strformat
-        call printf
-        add esp, 8
-        ret
+	push rbp
+	mov rbp, rsp
+
+	mov rdi, strformat
+	mov rsi, text
+	xor rax, rax
+	call printf
+
+	leave
+	ret
 ```
 
 Note that the procedure is declared as global and is called `main` - the starting point of any C program.

labs/lab-09/tasks/max-assembly-calls/solution/main.asm

@@ -37,6 +37,7 @@ main:
     mov rdi, fmt
     mov rsi, rax
     mov edx, dword [pos]
+    xor rax, rax
     call printf
 
     ; set exit code 0 (in main)

labs/lab-09/tasks/max-assembly-calls/support/main.asm

@@ -7,7 +7,12 @@ section .data
     arr: dd 19, 7, 129, 87, 54, 218, 67, 12, 19, 99
     len: equ $-arr
 
-    fmt: db "max: %u", 10, 0
+    fmt: db "max: %u on position: %u", 10, 0
+
+section .bss
+    ; we are _reserving_ space for a double word (4 bytes)
+    ; but we are not initializing it; so it can't reside in .data
+    pos: resd 1
 
 section .text
 
@@ -23,12 +28,16 @@ main:
     shr rsi, 2
 
     mov rdi, arr
+    mov rdx, pos
     call get_max
 
-    ; print maximum value
+    ; print maximum value and its position
     ; NOTE: RAX holds the return value of get_max()
+    ; NOTE: pos written by get_max() at given memory address
     mov rdi, fmt
     mov rsi, rax
+    mov edx, dword [pos]
+    xor rax, rax
     call printf
 
     ; set exit code 0 (in main)

labs/lab-09/tasks/max-c-calls/solution/max.asm

@@ -5,33 +5,46 @@ section .text
 global get_max
 
 
-; RDI = array pointer
-; RSI = array length
-; RDX = pos pointer
+; EDI = array pointer
+; ESI = array length
+; EDX = pos pointer
 get_max:
-    push rbp
-    mov  rbp, rsp
+    push ebp
+    mov  ebp, esp
+
+    ; save registers
+    push edi
+    push esi
+
+    mov edi, [ebp + 8]
+    mov esi, [ebp + 12]
+    mov edx, [ebp + 16]
+
 
     ; initialize EAX with the first value as currently known maximum
-    mov eax, [rdi]
-    mov [rdx], eax
+    mov eax, [edi]
+    mov dword [edx], 0
 
-    ; initialize RCX as loop counter for remaining elements
-    mov rcx, rsi
-    dec rcx
+    ; initialize ECX as loop counter for remaining elements
+    mov ecx, esi
+    dec ecx
 
     ; loop over remaining array elements
 compare:
-    cmp eax, [rdi + 4*rcx]
-    jge check_end
+    cmp eax, [edi + 4*ecx]
+    jae check_end
 
     ; update maximum and its position
-    mov eax, [rdi + 4*rcx]
-    mov [rdx], ecx
+    mov eax, [edi + 4*ecx]
+    mov [edx], ecx
 check_end:
     loop compare
 
-    ; result stored in RAX
+    ; result stored in EAX
+
+    ; restore registers
+    pop esi
+    pop edi
 
     leave
     ret

labs/lab-09/tasks/max-c-calls/support/max.asm

@@ -5,29 +5,36 @@ section .text
 global get_max
 
 
-; RDI = array pointer
-; RSI = array length
+; EDI = array pointer
+; ESI = array length
 get_max:
-    push rbp
-    mov  rbp, rsp
+    push ebp
+    mov  ebp, esp
+
+    ; save registers
+    push edi
+    push esi
+
+    mov edi, [ebp + 8]
+    mov esi, [ebp + 12]
 
     ; initialize EAX with the first value as currently known maximum
-    mov eax, [rdi]
+    mov eax, [edi]
 
-    ; initialize RCX as loop counter for remaining elements
-    mov rcx, rsi
-    dec rcx
+    ; initialize ECX as loop counter for remaining elements
+    mov ecx, esi
+    dec ecx
 
-    ; loop over remaining array elements
 compare:
-    cmp eax, [rdi + 4*rcx]
-    jge check_end
-    mov eax, [rdi + 4*rcx]
+    cmp eax, [edi + 4*ecx]
+    jae check_end
+    mov eax, [edi + 4*ecx]
 check_end:
     loop compare
 
-    ; result stored in RAX
+    ; restore registers
+    pop esi
+    pop edi
 
     leave
     ret
-

labs/lab-09/tasks/regs-preserve/solution/main.asm

@@ -44,12 +44,12 @@ next:
     mov rdi, newline
     call printf
 
-    ; restore preserved register
-    pop rbx
-
     ; restore the stack after calling printf
     add rsp, 8
 
+    ; restore preserved register
+    pop rbx
+
     leave
     ret
 

labs/lab-09/tasks/regs-preserve/support/main.asm

@@ -27,7 +27,8 @@ print_reverse_array:
     mov rcx, rsi
 
 next:
-    ; TODO1: uncomment the following two lines
+    ; TODO1: uncomment push rcx and pop rcx.
+    ; Note: pushing rcx also aligns the stack to 16 bytes for printf.
     ; push rcx
     xor rax, rax
     mov esi, [rbx + 4*rcx - 4]
@@ -40,6 +41,7 @@ next:
     mov rdi, newline
     call printf
 
+
     ; restore preserved register
     pop rbx