(gdb) added file for the GNU debugger

gdb.md

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+# GNU debugger
+
+<!--toc:start-->
+- [GNU debugger](#gnu-debugger)
+<!--toc:end-->
+
+
+## reset program
+
+To reset the program one can use the following command
+
+```sh
+# reset program and halt execution
+monitor reset halt
+```
+
+## TUI mode
+
+The `GDB` comes with a Terminal User Interface (TUI) which provides view for
+the assembly, the registers and the source code.
+
+One can enable the TUI with `tui enable` or disable with `tui disable`.
+
+```sh
+# enable/disable TUI
+tui enable
+tui disable
+```
+
+### TUI keybindings
+
+| **Keybinding** | **Action**             |
+|:---------------|:-----------------------|
+| `C-x,a/A/C-a`  | toggle TUI mode on/off |
+| `C-o`          | focus next window      |
+
+
+### navigation
+
+One can navigate using the [keybindings](#tui-keybindings), but the easiest
+method is to use the `focus` command.
+The `cmd` window is the only one that accepts alphanumeric input, so regardless
+of the focused window, one can always use `focus <window>` to focus a specific
+window.
+
+
+### layouts
+
+One can choose various layouts with the following command:
+
+```
+# choose layout, enable views
+layout <layout-name>
+# default layout options:
+# * asm
+# * regs
+# * src
+# * split
+```
+
+The `tui` command also allows to define custom layouts.
+See the example below.
+
+```
+# defines a layout with the name `test` and the view `regs`, `src` and `cmd`
+tui new-layout test {-horizontal regs 1 src 2} 1 cmd 1
+
+# synopsis of `tui new-layout`
+tui new-layout <name>  <view> <weight> [<view> <weight>]
+tui new-layout <name> {[-horizontal] <view> <weight> [<view> <weight>]} <weight>
+```