Clear screen via crt0.c (#38)

* Minimal OSI C1P implementation

All boilerplate except for the custom _exit implementation
that jumps to the PROM monitor prompt.

* __putchar() for OSI target

* Remove obsolete comment

* Switch to loop exit implementation

This does make more sense with the existing examples that
otherweise would jump directly to the PROM monitor.

* Switch to exit-loop implementation

The endless  loop in the abort implementation is
obsolete with the switch to the common-exit-loop
implementation.

* Address review comments

Use ' ' consistently for space character.
Fix incorrect encoding for CR and LF.

* C standard semantics of '\n'

* Attempt to implement crt0.c

This is a first broken attempt to hook up a call for clearing the screen
before entering main.

* Missing newline

* Fix problem with crt0.c

The CMake call for merge_libraries for common-crt0-o was missing.
This caused an invalid executable being generated where main itself
was missing.

* Adapt to new library merging strategy

It is no longer necessary to merge the crt0.o module explicitly
with the parent library crt0.o module.

* Fix incorrect argument for memset()

Fixed the bug that the address of function scr_base was passed
to memset() instead of calling scr_base() and passing the
result to memset().

* Fix C++ namespace pollution

Fix pollution of C++ namespace by class name that is
not prefixed by underscores.

Fixes #39.

* Simplify clear screen in crt0

Replaced crt0 C module containing inline assembly with
plain assembler module where __clrscr is called directly
without involving an additional C function.

mos-platform/osi-c1p/CMakeLists.txt

@@ -4,6 +4,8 @@ if(NOT CMAKE_CROSSCOMPILING)
   return()
 endif()
 
+add_platform_object_file(osi-c1p-crt0-o crt0.o crt0.s)
+
 add_platform_library(osi-c1p-crt0)
 merge_libraries(osi-c1p-crt0
   common-copy-data

mos-platform/osi-c1p/crt0.s

@@ -0,0 +1,6 @@
+; Assembler crt0 file to clear the screen before the
+; call to main.
+
+.section .init.40,"axR",@progbits
+  jsr __clrscr
+

mos-platform/osi-c1p/osi_screen.h

@@ -11,7 +11,7 @@ template <
     unsigned int screen_firstchar = 0x85, // offset of cursor position (0, 0) from base of video RAM
     unsigned int scroll_dist = 0x20 // memory distance for scrolling by one line
 >
-class osi_screen
+class __osi_screen
 {
 private:
     static unsigned char cursor_x;
@@ -58,7 +58,7 @@ class osi_screen
 public:
     static void clrscr(void)
     {
-        memset((void *) scr_base, ' ', video_ram_size);
+        memset((void *) scr_base(), ' ', video_ram_size);
         cursor_x = cursor_y = 0;
     }
 

mos-platform/osi-c1p/putchar.cc

@@ -4,17 +4,17 @@ template<unsigned int scr_base_int, unsigned int video_ram_size,
     unsigned int screen_width, unsigned int screen_height,
     unsigned int screen_firstchar, unsigned int scroll_dist>
     unsigned char
-    osi_screen<scr_base_int, video_ram_size, screen_width, screen_height,
+    __osi_screen<scr_base_int, video_ram_size, screen_width, screen_height,
          screen_firstchar, scroll_dist>::cursor_x;
 
 template<unsigned int scr_base_int, unsigned int video_ram_size,
     unsigned int screen_width, unsigned int screen_height,
     unsigned int screen_firstchar, unsigned int scroll_dist>
     unsigned char
-    osi_screen<scr_base_int, video_ram_size, screen_width, screen_height,
+    __osi_screen<scr_base_int, video_ram_size, screen_width, screen_height,
          screen_firstchar, scroll_dist>::cursor_y;
 
-using c1p_screen = osi_screen<>;
+using __osic1p_screen = __osi_screen<>;
 
 /**
  * @brief __putchar implementation for Challenger 1P
@@ -34,8 +34,17 @@ void __putchar(char c)
      */ 
     if (c == '\n')
     {
-        c1p_screen::cputc('\r');
+        __osic1p_screen::cputc('\r');
     }
 
-    c1p_screen::cputc(c);
+    __osic1p_screen::cputc(c);
+}
+
+/**
+ * @brief __clrscr clear the screen
+ */
+extern "C"
+void __clrscr(void)
+{
+    __osic1p_screen::clrscr();
 }