Add semihosting support (SerialSemi and SemiFS)

docs/index.rst

@@ -43,6 +43,7 @@ For the latest version, always check https://github.com/earlephilhower/arduino-p
    File Systems (SD, SDFS, LittleFS) <fs>
    USB (Arduino and Adafruit_TinyUSB) <usb>
    Multicore Processing <multicore>
+   Semihosting <semihosting>
 
    RP2350 Specific Notes <rp2350>
    RP2350 PSRAM <psram>

docs/semihosting.rst

@@ -0,0 +1,56 @@
+Semihosting Support
+===================
+
+Using special debugger breakpoints and commands, the Pico can read and write to the debugging console as well
+as read and write files on a development PC.  The ``Semihosting`` library allows applications to use the
+semihosting support as a normal filesystem or serial port.
+
+**NOTE** Semihosting only works when connected to an OpenOCD + GDB debug session.  Running an application
+compiled for Semihosting without the debugger will cause a panic and hang the chip.
+
+As of now, only ARM has support for Semihosting.
+
+Running Semihosting on the Development Host
+-------------------------------------------
+
+Start OpenOCD normally from inside a directory that you can read and write files within (i.e. do not run from
+``C:\\Program Files\\..`` on Windows where general users aren't allowed to write).  The starting
+directory will be where the Pico will read and write files using the ``SemiFS`` class.
+Be sure to keep the terminal window you ran OpenOCD in open, because all ``SerialSemi`` input and output
+will go to **that** terminal and not ``gdb``'s.
+
+Start GDB normally and connect to the OpenOCD debugger and enable semihosting support
+
+.. code::
+
+    (gdb) target extended-remote localhost:3333
+    (gdb) monitor arm semihosting enable
+
+At this point load and run your ``ELF`` application as normal.  Again, all ``SerialSemi`` output will go
+to the **OpenOCD** window, not GDB.
+
+See the ``hellosemi`` example in the ``Semihosting`` library.
+
+SerialSemi - Serial over Semihosting
+------------------------------------
+
+Simply include ``<Semihosting.h>`` in your application and use ``SerialSemi`` as you would any other
+``Serial`` port with the following limitations:
+
+* Baud rate, bit width, etc. are all ignored
+* Input is limited because ``read`` may hang indefinitely in the host and ``available`` is not part of the spec
+
+SemiFS - Host filesystem access through Semihosting
+---------------------------------------------------
+
+Use ``SemiFS`` the same way as any other file system.  Note that only file creation and renaming are supported, with
+no provision for iterating over directories or listing files.  In most cases simply opening a ``File`` and writing out
+a debug dump is all that's needed:
+
+.. code::
+
+    SemiFS.begin();
+    File f = SemiFS.open("debug.dmp", "w");
+    f.write(buffer, size);
+    f.close();
+    SerialSemi.printf("Debug dump nopw available on host.\n");

libraries/Semihosting/examples/hellosemi/hellosemi.ino

@@ -0,0 +1,43 @@
+// This example uses semihosting to send serial output to the OpenOcD screen
+// and write binary files to the host system.
+//
+// Semihosting **ONLY** works with an OpenOCD and GDB setup.  If you build
+// and run a semihosting app without GDB connected, it **WILL CRASH**
+//
+// Start OpenOCD normally, but leave the terminal window visible because
+// is it OpenOCD, not GDB, which will display the semihosting output.
+// OpenOCD will also create files in the current working directory, so
+// be sure it is a place you can find and write to.
+//
+// In GDB,connect to OpenOCD and then enable semihosting
+// (gdb) target extended-remote localhost:3333
+// (gdb) monitor arm semihosting enable
+
+#ifdef __riscv
+void setup() {
+  // No semihosting for RISCV yet
+}
+void loop() {
+}
+#else
+
+#include <Semihosting.h>
+
+int c = 0;
+
+void setup() {
+  SerialSemi.begin();
+  SerialSemi.printf("HELLO, GDB!\n");
+  SemiFS.begin();
+  File f = SemiFS.open("out.bin", "w");
+  f.printf("I made a file!\n");
+  f.close();
+  SerialSemi.printf("Just wrote a file 'out.bin'\n");
+}
+
+void loop() {
+  SerialSemi.printf("SH Loop Count: %d\n", c++);
+  Serial.printf("USB Loop Count: %d\n", c++);
+  delay(1000);
+}
+#endif

libraries/Semihosting/keywords.txt

@@ -0,0 +1,18 @@
+#######################################
+# Syntax Coloring Map
+#######################################
+
+#######################################
+# Datatypes (KEYWORD1)
+#######################################
+
+SerialSemi	KEYWORD1
+SemiFS	KEYWORD1
+
+#######################################
+# Methods and Functions (KEYWORD2)
+#######################################
+
+#######################################
+# Constants (LITERAL1)
+#######################################

libraries/Semihosting/library.properties

@@ -0,0 +1,10 @@
+name=Semihosting
+version=1.0.0
+author=Earle F. Philhower, III <[email protected]>
+maintainer=Earle F. Philhower, III <[email protected]>
+sentence=Semihosted serial and filesystem access for the Pico and OpenOCD
+paragraph=Semihosted serial and filesystem access for the Pico and OpenOCD
+category=Communications
+url=https://github.com/earlephilhower/arduino-pico
+architectures=rp2040
+dot_a_linkage=true