stack/functions: Add functions lab.

chapters/stack/functions/drills/tasks/print_rev_string/README.md

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+# Displaying the Reversed String
+
+In the file `print_rev_string.asm`, add the `reverse_string` function so that you have a listing similar to the one below:
+
+```Assembly
+[...]
+section .text
+global main
+
+reverse_string:
+    push ebp
+    mov ebp, esp
+
+    mov eax, dword [ebp+8]
+    mov ecx, dword [ebp+12]
+    add eax, ecx
+    dec eax
+    mov edx, dword [ebp+16]
+
+copy_one_byte:
+    mov bl, byte [eax]
+    mov byte [edx], bl
+    dec eax
+    inc edx
+    loopnz copy_one_byte
+
+    inc edx
+    mov byte [edx], 0
+
+    leave
+    ret
+
+main:
+    push ebp
+    mov ebp, esp
+[...]
+```
+
+> **IMPORTANT:**  When copying the `reverse_string` function into your program, remember that the function starts at the `reverse_string` label and ends at the `main` label. The `copy_one_byte` label is part of the `reverse_string` function.
+
+The `reverse_string` function reverses a string and has the following signature: `void reverse_string(const char *src, size_t len, char *dst);`. This means that the first `len` characters of the `src` string are reversed into the `dst` string.
+
+Reverse the `mystring` string into a new string and display that new string.
+
+> **NOTE:**  To define a new string, we recommend using the following construction in the data section:
+>
+> ```Assembly
+> store_string times 64 db 0
+> ```
+>
+> This creates a string of 64 zero bytes, enough to store the reverse of the string.
+> The equivalent C function call is `reverse_string(mystring, ecx, store_string);`. We assume that the length of the string is calculated and stored in the `ecx` register.
+>
+> You cannot directly use the value of `ecx` in its current form. After the `printf` function call for displaying the length, the value of `ecx` is lost. To retain it, you have two options:
+>
+> 1. Store the value of the `ecx` register on the stack beforehand (using `push ecx` before the `printf` call) and then restore it after the `printf` call (using `pop ecx`).
+> 2. Store the value of the `ecx` register in a global variable, which you define in the `.data` section.
+>
+> You cannot use another register because there is a high chance that even that register will be modified by the `printf` call to display the length of the string.

chapters/stack/functions/drills/tasks/print_rev_string/solution/Makefile

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+../support/Makefile

chapters/stack/functions/drills/tasks/print_rev_string/solution/print_reverse_string.asm

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+section .data
+    mystring db "This is my string", 0
+    print_format db "String length is %d", 10, 0
+    store_string times 64 db 0
+
+section .text
+
+extern printf
+extern puts
+global main
+
+reverse_string:
+    push ebp
+    mov ebp, esp
+
+    mov eax, dword [ebp+8]
+    mov ecx, dword [ebp+12]
+    add eax, ecx
+    dec eax
+    mov edx, dword [ebp+16]
+
+copy_one_byte:
+    mov bl, byte [eax]
+    mov byte [edx], bl
+    dec eax
+    inc edx
+    loopnz copy_one_byte
+
+    inc edx
+    mov byte [edx], 0
+
+    leave
+    ret
+
+main:
+    push ebp
+    mov ebp, esp
+
+    mov eax, mystring
+    xor ecx, ecx
+test_one_byte:
+    mov bl, byte [eax]
+    test bl, bl
+    jz out
+    inc eax
+    inc ecx
+    jmp test_one_byte
+
+out:
+    ; save ecx's value since it can be changed by printf
+    push ecx
+
+    push ecx
+    push print_format
+    call printf
+    add esp, 8
+
+    pop ecx
+
+    push store_string
+    push ecx
+    push mystring
+    call reverse_string
+    add esp, 12
+
+    push store_string
+    call puts
+    add esp, 4
+
+    leave
+    ret

chapters/stack/functions/drills/tasks/print_rev_string/support/Makefile

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+AS = nasm
+CC = gcc
+RM = rm
+
+SRCS := $(shell find . -name "*.asm")
+OBJS := $(SRCS:.asm=.o)
+
+UTILSDIR := ../utils/
+
+ASFLAGS ?= -f elf32 -F dwarf -I "$(UTILSDIR)"
+CFLAGS ?= -Wall
+LDFLAGS ?= -m32 -no-pie
+
+all: print_reverse_string
+
+print_reverse_string_: print_reverse_string.o
+	$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
+
+%.o: %.asm
+	$(AS) $(ASFLAGS) $< -o $@
+
+.PHONY: clean
+
+clean:
+	$(RM) -r *.o print_reverse_string

chapters/stack/functions/drills/tasks/print_rev_string/support/print_reverse_string.asm

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+%include "../utils/printf32.asm"
+
+section .data
+    mystring db "This is my string", 0
+
+section .text
+
+extern puts
+extern printf
+global main
+
+main:
+    push ebp
+    mov ebp, esp
+
+    mov eax, mystring
+    xor ecx, ecx
+test_one_byte:
+    mov bl, byte [eax]
+    test bl, bl
+    je out
+    inc eax
+    inc ecx
+    jmp test_one_byte
+
+out:
+    PRINTF32 `[before]: %s\n[after]: \x0`, mystring
+
+    ; TODO: print reverse string
+
+    leave
+    ret

chapters/stack/functions/drills/tasks/print_rev_string/utils/printf32.asm

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+; SPDX-License-Identifier: BSD-3-Clause
+
+;;; macro to use printf with 32bit parameters:
+;;; - 1st parameter MUST be an immediate in backquotes `EAX=%d ECX=%x \n\x0`
+;;;     escape \n and \x0 only work with backquotes
+;;; - rest of parameters MUST be 32bit
+;;; - gen purpose and flags are preserved
+;;; - stack is cleaned
+%macro PRINTF32 1-*
+    pushf
+    pushad
+    jmp     %%endstr
+%%str:       db      %1
+%%endstr:
+%rep  %0 - 1
+%rotate -1
+    push    dword %1
+%endrep
+    push %%str
+    call printf
+    add esp, 4*%0
+    popad
+    popf
+%endmacro

chapters/stack/functions/drills/tasks/rot13/README.md

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+# Bonus: Rot13
+
+Create and use a function that performs [rot13](https://rot13.com/) translation of a string.
+
+# Bonus: Rot13++
+
+Implement `rot13` on an array of strings: the strings are contiguous in memory separated by the string terminator (`NULL`-byte, `0`). For example, `ana\0are\0mere\0` is an array of three strings.
+
+Apply `rot13` to alphabetical characters and replace the string terminator with a space (`' '`, blank, character `32`, or `0x20`). Thus, the initial string `ana\0are\0mere\0` will translate to `nan ner zrer`.
+
+> **NOTE:**  To define the array of strings containing the string terminator, use a construction like:
+>
+> ```Assembly
+> mystring db "ana", 0, "are", 0, "mere", 0
+> ```
+
+> **NOTE:**  You will need to know when to stop traversing the array of strings. The simplest way is to define a length variable in the `.data` section, like so:
+>
+> ```Assembly
+> len dd 10
+> ```
+>
+> where you either store the total length of the string (from the beginning to the last `NULL` byte) or the number of strings in the array.

chapters/stack/functions/drills/tasks/string_print/README.md

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+# Displaying a String
+
+To display a string, we can use the internal macro `PRINTF32`. Alternatively, we can use a function such as `puts`. In the file `print_string.asm`, displaying a string using the `PRINTF32` macro is implemented.
+
+Following the example of the `hello_world.asm` file, implement string display using `puts` as well.
+
+
+If you're having difficulties solving this exercise, take a peek at [hello_world.asm](../../../guides/hello_world/).

chapters/stack/functions/drills/tasks/string_print/solution/Makefile

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+../support/Makefile

chapters/stack/functions/drills/tasks/string_print/solution/print_string.asm

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+%include "../utils/printf32.asm"
+
+section .data
+    mystring db "This is my string", 0
+
+section .text
+
+extern puts
+extern printf
+global main
+main:
+    push ebp
+    mov ebp, esp
+
+    PRINTF32 `[PRINTF32]: %s\n[PUTS]: \x0`, mystring
+
+
+    push mystring
+    call puts
+    add esp, 4
+
+    leave
+    ret

chapters/stack/functions/drills/tasks/string_print/support/Makefile

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+AS = nasm
+CC = gcc
+RM = rm
+
+SRCS := $(shell find . -name "*.asm")
+OBJS := $(SRCS:.asm=.o)
+
+UTILSDIR := ../utils/
+
+ASFLAGS ?= -f elf32 -F dwarf -I "$(UTILSDIR)"
+CFLAGS ?= -Wall
+LDFLAGS ?= -m32 -no-pie
+
+all: print_string
+
+print_string: print_string.o
+	$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
+
+%.o: %.asm
+	$(AS) $(ASFLAGS) $< -o $@
+
+.PHONY: clean
+
+clean:
+	$(RM) -r *.o print_string

chapters/stack/functions/drills/tasks/string_print/support/print_string.asm

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+%include "../utils/printf32.asm"
+
+section .data
+    mystring db "This is my string", 0
+
+section .text
+
+extern puts
+extern printf
+global main
+main:
+    push ebp
+    mov ebp, esp
+
+    PRINTF32 `[PRINTF32]: %s\n[PUTS]: \x0`, mystring
+
+
+    ; TODO: call puts on string
+
+    leave
+    ret

chapters/stack/functions/drills/tasks/string_print/utils/printf32.asm

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+; SPDX-License-Identifier: BSD-3-Clause
+
+;;; macro to use printf with 32bit parameters:
+;;; - 1st parameter MUST be an immediate in backquotes `EAX=%d ECX=%x \n\x0`
+;;;     escape \n and \x0 only work with backquotes
+;;; - rest of parameters MUST be 32bit
+;;; - gen purpose and flags are preserved
+;;; - stack is cleaned
+%macro PRINTF32 1-*
+    pushf
+    pushad
+    jmp     %%endstr
+%%str:       db      %1
+%%endstr:
+%rep  %0 - 1
+%rotate -1
+    push    dword %1
+%endrep
+    push %%str
+    call printf
+    add esp, 4*%0
+    popad
+    popf
+%endmacro

chapters/stack/functions/drills/tasks/string_print_len/README.md

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+# Displaying the Length of a String
+
+The program `print_string_len.asm` displays the length of a string using the `PRINTF32` macro. The calculation of the length of the `mystring` string occurs within the program (it is already implemented).
+
+Implement the program to display the length of the string using the `printf` function.
+
+At the end, you will have the length of the string displayed twice: initially with the `PRINTF32` macro and then with the external function call `printf`.
+
+> **NOTE:**  Consider that the `printf` call is of the form `printf("String length is %u\n", len);`. You need to construct the stack for this call.
+>
+> The steps to follow are:
+>
+> 1. Mark the symbol `printf` as external.
+> 2. Define the format string `"String length is %u", 10, 0`.
+> 3. Make the function call to `printf`, i.e.:
+>     1. Put the two arguments on the stack: the format string and the length.
+>     2. Call `printf` using `call`.
+>     3. Restore the stack.
+>
+> The length of the string is found in the `ecx` register.

chapters/stack/functions/drills/tasks/string_print_len/solution/Makefile

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+../support/Makefile