semihosting: initial implementation of ARM semihosting

Useful for logging output to the host console.

semihosting/semihosting.go

@@ -0,0 +1,47 @@
+// Package semihosting implements parts of the ARM semihosting specification,
+// for communicating over a debug connection.
+//
+// If you want to use it in OpenOCD, you have to enable it first with the
+// following command:
+//
+//     arm semihosting enable
+package semihosting
+
+import (
+	"device/arm"
+	"unsafe"
+)
+
+// IOError is returned by I/O operations when they fail.
+type IOError struct {
+	BytesWritten int
+}
+
+func (e *IOError) Error() string {
+	return "semihosting: I/O error"
+}
+
+// Write writes the given data to the given file descriptor. It returns an
+// *IOError if the write was not successful.
+func Write(fd uintptr, data []byte) error {
+	if len(data) == 0 {
+		return nil
+	}
+	params := struct {
+		fd   uintptr
+		data unsafe.Pointer
+		len  int
+	}{
+		fd:   fd,
+		data: unsafe.Pointer(&data[0]),
+		len:  len(data),
+	}
+	unwritten := arm.SemihostingCall(arm.SemihostingWrite, uintptr(unsafe.Pointer(&params)))
+	if unwritten != 0 {
+		// Error: unwritten is the number of bytes not written.
+		return &IOError{
+			BytesWritten: len(data) - unwritten,
+		}
+	}
+	return nil
+}

semihosting/stdio.go

@@ -0,0 +1,20 @@
+package semihosting
+
+// These three file descriptors are connected to the host stdin/stdout/stderr,
+// and can be used for logging.
+var (
+	Stdin  = File{fd: 0}
+	Stdout = File{fd: 1}
+	Stderr = File{fd: 2}
+)
+
+// File represents a semihosting file descriptor.
+type File struct {
+	fd uintptr
+}
+
+// Write writes the given data buffer to the file descriptor, returning an error
+// if the write could not complete successfully.
+func (f *File) Write(buf []byte) error {
+	return Write(f.fd, buf)
+}