On Arch Linux, install:

    arm-none-eabi-gcc
    arm-none-eabi-gdb
    qemu-arch-extra

To build the `present` binary, run:

    make present

To start debugging it, run:

    make run-present

# Example debugging session
This section shows how to debug using GDB.

    exercises$ make run-present
    qemu-arm -g 1337 present &
    arm-none-eabi-gdb -q -ex 'tar rem localhost:1337' present
    Reading symbols from present...done.
    Remote debugging using localhost:1337
    _start () at present.S:58
    58          ldr r0, =input

Step a single instruction:

    (gdb) stepi
    59          ldr r1, =sbox

Press "Enter" to repeat the last command (stepi):

    (gdb) 
    60          bl  sub_block

Examine 16 bytes of memory from the address in register `$r0` (our "input"):

    (gdb) x/16b $r0
    0x2000: 5       1       2       3       4       5       6       7
    0x2008: 8       9       10      11      12      13      14      15

Examine 16 bytes of memory as hex from the address in register `$r1` (our "sbox"):

    (gdb) x/16x $r1
    0x2010: 0x0c    0x05    0x06    0x0b    0x09    0x00    0x0a    0x0d
    0x2018: 0x03    0x0e    0x0f    0x08    0x04    0x07    0x01    0x02
    (gdb) stepi
    sub_block () at present.S:30
    30          eor r3, r3

Set a breakpoint on line 38, continue until that breakpoint:

    (gdb) break 38
    Breakpoint 1 at 0x1014: file present.S, line 38.
    (gdb) c
    Continuing.

    Breakpoint 1, sub_block_loop () at present.S:38
    38          and r4, #15

List the source code around the current line:

    (gdb) l
    33          bge sub_block_loop_end
    34
    35          // c = input[i] (r4 is low, r5 is high)
    36          ldrb r4, [r0, r3]
    37          mov r5, r4
    38          and r4, #15
    39          lsr r5, #4
    40
    41          // sbox[c_lo]
    42          ldrb r4, [r1, r4]

Print the contents of some register (normally in decimal, using the `/x`
specifier it becomes hexadecimal):

    (gdb) p $r0
    $1 = 8192
    (gdb) p/x $r0
    $2 = 0x2000
    (gdb) p/x $r3
    $3 = 0x0
    (gdb) p/x $r4
    $4 = 0x5
    (gdb) x/b $r0+$r3
    0x2000: 0x05

A quick overview of all registers:

    (gdb) info registers
    r0             0x2000   8192
    r1             0x2010   8208
    r2             0x0      0
    r3             0x0      0
    r4             0x5      5
    r5             0x5      5
    r6             0x0      0
    r7             0x0      0
    r8             0x0      0
    r9             0x0      0
    r10            0x0      0
    r11            0x0      0
    r12            0x0      0
    sp             0xf6fff5e0       0xf6fff5e0
    lr             0x1044   4164
    pc             0x1014   0x1014 <sub_block_loop+16>
    cpsr           0x80000010       -2147483632

Exiting the debugger:

    (gdb) quit
    A debugging session is active.

            Inferior 1 [Remote target] will be killed.

    Quit anyway? (y or n) y

    QEMU: Terminated via GDBstub