### 13.2 Existing Targets

#### 13.2.1 File Targets

Both executables and core files have target vectors.

#### 13.2.2 Standard Protocol and Remote Stubs

gdb's file `remote.c` talks a serial protocol to code that runs in the target system. gdb provides several sample stubs that can be integrated into target programs or operating systems for this purpose; they are named `cpu-stub.c`. Many operating systems, embedded targets, emulators, and simulators already have a gdb stub built into them, and maintenance of the remote protocol must be careful to preserve compatibility.

The gdb user's manual describes how to put such a stub into your target code. What follows is a discussion of integrating the SPARC stub into a complicated operating system (rather than a simple program), by Stu Grossman, the author of this stub.

The trap handling code in the stub assumes the following upon entry to `trap_low`:

1. %l1 and %l2 contain pc and npc respectively at the time of the trap;
2. traps are disabled;
3. you are in the correct trap window.

As long as your trap handler can guarantee those conditions, then there is no reason why you shouldn't be able to “share” traps with the stub. The stub has no requirement that it be jumped to directly from the hardware trap vector. That is why it calls `exceptionHandler()`, which is provided by the external environment. For instance, this could set up the hardware traps to actually execute code which calls the stub first, and then transfers to its own trap handler.

For the most point, there probably won't be much of an issue with “sharing” traps, as the traps we use are usually not used by the kernel, and often indicate unrecoverable error conditions. Anyway, this is all controlled by a table, and is trivial to modify. The most important trap for us is for `ta 1`. Without that, we can't single step or do breakpoints. Everything else is unnecessary for the proper operation of the debugger/stub.

From reading the stub, it's probably not obvious how breakpoints work. They are simply done by deposit/examine operations from gdb.

#### 13.2.3 ROM Monitor Interface

#### 13.2.4 Custom Protocols

#### 13.2.5 Transport Layer

#### 13.2.6 Builtin Simulator