Modifying the Example Scenario

Modifying the Example Scenario#

Now that we built and executed the example scenario that continuously writes “Hello World!” to the console, we are going to adjust that scenario. The build method you used for your first build is independent of this and the following guide. When asked to recompile and run the scenario, just use the commands suited for the respective build tool:

cd $BOB_DIR
bob dev --dev-sandbox examples-amd64/examples::hello -bj
dev/dist/examples/hello/1/workspace/bootx64.efi.launch

Note

The -b option (--build-only) will skip the update of the L4Re git repositories. This speeds up the build.

cd $L4RE_OBJDIR
make -j9
make E=hello-cfg qemu MODULE_SEARCH_PATH=$KERNEL_OBJDIR:$L4RE_SRCDIR/conf/examples

What Runs Below Hello#

When executing the example scenario hello-cfg, what actually happens is the following:

  • The L4Re Microkernel is started and launches the two initial application that manages memory and the first application that is responsible for providing an environment to launch the rest of the system.

  • Sigma0 manages memory. Sigma0 is started and gets the necessary memory information from the kernel. With this information, the application responsible for starting applications can be started.

  • Moe starts user applications and provides a environment with convenient abstractions to them. Moe is started and provides abstractions for managing memory, logging and constraining scheduling parameters. It can start another arbitrary application that can now rely on these abstractions.

  • Ned is comparable to the init system of a POSIX system. It can conveniently be configured by users through a Lua scripting interface and brings up the rest of the applications.

    The Lua script specifying the scenario configuration to be launched by Ned is stored in $BOB_DIR/recipes/example/hello/hello.cfg, which initially just starts the hello application.

    The Lua script specifying the scenario configuration to be launched by Ned is stored in $L4RE_SRCDIR/conf/example/hello.cfg, which initially just starts the hello application.

Inspecting The Scenario#

The hello.cfg file is a Lua script with the following content:

1-- vim:ft=lua
2-- this is a configuration to start 'hello'
3
4local L4 = require("L4");
5
6L4.default_loader:start({}, "rom/hello");

The Lua functionality for configuring a scenario and interfacing with the L4 system functionality is in the L4 Module, which is included in line 4. This module holds a default application loader, that is instructed to start the application known under the name rom/hello.

There is another file that is important for this scenario to work as expected: $BOB_DIR/recipes/examples/hello/modules.list:

1entry hello-cfg
2kernel fiasco -serial_esc
3roottask moe rom/hello.cfg
4module l4re
5module ned
6module hello.cfg
7module hello

There is another file that is important for this scenario to work as expected: $L4RE_SRCDIR/conf/modules.list:

66entry hello-cfg
67kernel fiasco -serial_esc
68roottask moe rom/hello.cfg
69module l4re
70module ned
71module hello.cfg
72module hello

It describes to composition of a system to be booted. Every module line specifies a binary that is to be made available inside the system in the rom namespace (comparable to a directory) which is accessible inside Ned.

The third line tells Moe which scenario to hand over to Ned to be interpreted. Moe defaults to running the binary named ned unless otherwise instructed.

A second Hello#

Back to the scenario file, we can now start a second instance of the hello example application by duplicating the line that starts the first one:

1-- vim:ft=lua
2-- this is a configuration to start 'hello'
3
4local L4 = require("L4");
5
6L4.default_loader:start({}, "rom/hello");
7L4.default_loader:start({}, "rom/hello");

Hint

Building with Gnu Make, there is no recompilation needed to run this altered scenario, since it is interpreted at run time.

Running this altered scenario will output double the amount of “Hello World!” lines to the console. Though, needing to count the number of lines per second can be a tedious task just to confirm the scenario does exactly as we want. So let’s use the empty pair of braces, we blissfully ignored until now:

1-- vim:ft=lua
2-- this is a configuration to start 'hello'
3
4local L4 = require("L4");
5
6L4.default_loader:start({ log = { "hello-1", "red" } }, "rom/hello");
7L4.default_loader:start({ log = { "hello-2", "cyan" } }, "rom/hello");

With this change, we configured the console logger to use different colors for the output of both hello applications. We also configured different prefixes.

Note

The different colors and prefixes are not processed in the hello application. Instead, the output of those applications is colored by the logger.

Next steps#

Change hello itself to take cmd line arguments

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