Improve ASM and C examples.

Added more comments to explain what was going on. I also expanded the C example to use newlib and show how malloc works.

Author: thejpster

samples/asmhello/asmhello.S

@@ -1,35 +1,60 @@
+/**
+ * Basic ARM Unified Assembly Language sample which runs on Neotron OS.
+ */
+
 .syntax unified
-.thumb
-.cpu cortex-m0plus
-.global app_entry
-.thumb_func
+
+//
+// Functions (.text)
+//
 
 .text
-.section ".text"
 
-// r0 will contain a pointer to the syscall table
-// the write function is the third entry (offset 8)
-// and takes the arguments r0=handle, r1=pointer, r2=length
+// Entry point for our application.
+//
+// * r0 will contain a pointer to the syscall table.
+// * The write function is the third entry (offset 8)
+//   and takes the arguments r0=handle, r1=pointer, r2=length
+.thumb_func
+.global app_entry
+.func app_entry
 app_entry:
-    // Save registers
-    push {r0, r1, r2, lr}
+    // "A subroutine must preserve the contents of the registers r4-r11
+    // and SP" - and we only use R4
+    push {r4, lr}
     // Fetch write function address
     ldr r3, [r0, #8]
-    // Set up data length
-    movs r2, #13
     // Set up file handle
     movs r0, #1
     // Set up data pointer
     ldr r1, =message
+    // Set up data length
+    ldr r4, =message_len
+    ldr r2, [r4]
     // Call write function
     blx r3
     // Set return value
     movs r0, #0
     // Exit
-    pop {r1, r2, r3, pc}
+    pop {r4, pc}
+.endfunc
+
+//
+// Read Only Data (.rodata)
+//
 
-.data
-.section ".rodata"
+.section .rodata
 
+// The message we want to print
+.type message,%object
 message:
 .ascii "Hello world!\n"
+
+// The length of the string with the label `message`
+//
+// Must come immediately after `message:`
+.type message_len,%object
+message_len:
+.long . - message  
+
+// End of file

samples/asmhello/build.sh

@@ -1,2 +1,8 @@
 #!/bin/sh
-arm-none-eabi-gcc -mcpu=cortex-m0 -Wl,-T../neotron-cortex-m.ld -o asmhello.elf asmhello.S -nostdlib
+arm-none-eabi-gcc \
+    -nostartfiles \
+    -ffreestanding \
+    -mcpu=cortex-m0plus \
+    -Wl,-T../neotron-cortex-m.ld \
+    -o asmhello.elf \
+    asmhello.S \

samples/chello/.gitignore

@@ -1 +1,3 @@
 *.elf
+*.o
+

samples/chello/build.sh

@@ -1,2 +1,22 @@
 #!/bin/sh
-arm-none-eabi-gcc -Os -mcpu=cortex-m0 -Wl,-T../neotron-cortex-m.ld -o chello.elf chello.c -nostdlib
+
+arm-none-eabi-gcc \
+    -fdata-sections \
+    -ffreestanding \
+    -ffunction-sections \
+    -flto \
+    -mcpu=cortex-m0 \
+    -nostartfiles \
+    -Os \
+    -Wall \
+    -Wconversion \
+    -Wdouble-promotion \
+    -Wextra \
+    -Wl,-gc-sections \
+    -Wl,-T../neotron-cortex-m.ld \
+    -Wshadow \
+    --specs=nano.specs \
+    --specs=nosys.specs \
+    -o chello.elf \
+    stubs.c \
+    chello.c

samples/chello/chello.c

@@ -3,17 +3,37 @@
  */
 
 #include "neotron.h"
+#include <stdio.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
-int app_entry(const NeotronApi *f)
-{
-    FfiByteSlice buffer = {
-        .data = "Hello, world",
-        .data_len = 12
-    };
-    Handle fd = {
-        // fd 1 is STDOUT
-        ._0 = 1
-    };
-    f->write(fd, buffer);
-    return 0;
+const NeotronApi *g_api;
+
+static int main(void);
+
+/*
+ * Called by Neotron OS when the binary is 'run'.
+ */
+int app_entry(const NeotronApi *f) {
+  g_api = f;
+  return main();
 }
+
+/*
+ * Our main function.
+ *
+ * Just prints a message and exits.
+ */
+static int main(void) {
+  // allocate a buffer
+  void *buffer = calloc(1024, 1);
+  // write a string into it
+  snprintf(buffer, 1023, "Hello, world!\n");
+  // print the buffer
+  printf(buffer);
+  // free the buffer
+  free(buffer);
+  return 0;
+}
+
+// End of file

samples/chello/stubs.c

@@ -0,0 +1,35 @@
+/*
+ * Newlib stub functions we need.
+ */
+
+#include <stdint.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "neotron.h"
+
+extern const NeotronApi *g_api;
+
+/*
+ * Implementation of the newlib library syscall `write`
+ */
+int _write(int fd, const void *data, size_t count) {
+  if (fd >= 255) {
+    return -1;
+  }
+  FfiByteSlice buffer = {
+      .data = data,
+      .data_len = count,
+  };
+  Handle neo_fd = {
+      ._0 = (uint8_t)fd,
+  };
+  FfiResult_void result = g_api->write(neo_fd, buffer);
+  if (result.tag == FfiResult_Ok) {
+    return (int)count;
+  } else {
+    return -1;
+  }
+}
+
+// End of file

samples/neotron-cortex-m.ld

@@ -20,6 +20,7 @@ MEMORY
 }
 
 /* # Entry point = what the BIOS calls to start the OS */
+EXTERN(app_entry);
 ENTRY(app_entry);
 
 /* # Sections */
@@ -31,42 +32,42 @@ SECTIONS
     . = ALIGN(4);
     *(.text .text.*);
     . = ALIGN(4);
-  }
+  } > RAM
 
   /* ### .rodata */
   .rodata : ALIGN(4)
   {
     . = ALIGN(4);
     *(.rodata .rodata.*);
     . = ALIGN(4);
-  }
+  } > RAM
 
   /* ### .data */
   .data : ALIGN(4)
   {
     . = ALIGN(4);
     *(.data .data.*);
     . = ALIGN(4);
-  }
-
-  /* LMA of .data */
-  __sidata = LOADADDR(.data);
+  } > RAM
 
   /* ### .bss */
   .bss : ALIGN(4)
   {
     . = ALIGN(4);
     *(.bss .bss.*);
     . = ALIGN(4);
-  }
+  } > RAM
 
   /* ### .uninit */
   .uninit (NOLOAD) : ALIGN(4)
   {
     . = ALIGN(4);
     *(.uninit .uninit.*);
     . = ALIGN(4);
-  }
+  } > RAM
+
+  . = ALIGN(4);
+  end = .;
 
   /* ## .got */
   /* Dynamic relocations are unsupported. This section is only used to detect relocatable code in