updated README with more porting information

Author: John Buyi Yu

README.md

@@ -2,3 +2,98 @@
 
 This is the portable code of MRTOS for the ARM Cortex M0+ platform. To use this
 platform, please see [the main mRTOS repository](https://github.com/jdoe95/mrtos). 
+
+The portable code uses the following interrupts:
+	* SVCall, ``isr_svc``, used for loading the first thread
+	* SysTick, ``isr_systick``, used for generating heart beat
+	* PendSV, ``isr_pendsv``, implemented as the context switcher
+
+The portable code does not configure the SYSTICK timer. The configuration of 
+SYSTICK must be set before os_start(), and __SYSTICK must be disabled__. When
+the operating system starts, it will enable the timer automatically.
+
+Do not use ``CPSID I`` and ``CPSIE I`` instructions in the application. 
+Use the nested version provided by the operating system instead (``os_enter_critical()`` and ``os_exit_critical()``). Be sure to
+replace the macros in CMSIS, too, if it's being used. Manipulating interrupts directly in the application can really mess up the system's shared variable access locks.
+
+## Features
+  * Low power mode (when idle)
+  
+## Settings
+
+| Setting | Value |
+|---|---|
+|SYSTICK interrupt frequency| 100 Hz (recommended)|
+|SYSTICK priority| highest (recommended) |
+|SVC priority | highest (required) |
+|PendSV priority| lowest (recommended) |
+  
+## Code example
+
+
+```C
+/*
+ * These variables are defined by the linker as
+ * instructed by the linker script. 
+ *
+ * Example linker script output section (GCC), place
+ * it after all other RAM sections.
+ *
+ * .blank_ram : ALIGN(4) {
+ *	   PROVIDE( _l_blank_ram_start = . );
+ *   // automatically adjust the size of the hole
+ *   // to use all remaining RAM
+ * 	. = ORIGIN(INT_SRAM) + LENGTH(INT_SRAM);
+ * 	PROVIDE( _l_blank_ram_end = . );
+ * } >INT_SRAM
+ *
+ */
+extern int _l_blank_ram_end;
+extern int _l_blank_ram_start;
+
+int main(void)
+{
+	// initialize hardware, configure SYSTICK, enable SYSTICK interrupt, 
+	// but do not enable counting.
+	// P.S. the CMSIS function will enable SYSTICK immediately. You can 
+	// either modify it or write your own.
+	
+	// SYSTICK has two input clocks, the CPU clock HCLK, or an external clock
+	systick_config( SYSTICK_CLOCK_SOURCE_CPU, CPU_CLOCK_FREQ/100);
+	
+	// set exception priorities
+	scb_set_svcall_prio(IRQ_PRIO_0);
+	scb_set_systick_prio(IRQ_PRIO_0);
+	scb_set_pendsv_prio(IRQ_PRIO_3);
+	
+	// ...
+	
+	// Initialize the operating system
+
+	os_config_t os_config;
+
+	os_config.p_pool_mem = &_l_blank_ram_start;
+	os_config.pool_size = (uint8_t*)&_l_blank_ram_end -  (uint8_t*)&_l_blank_ram_start;
+
+	os_init( &os_config );
+	
+	// create some operating system objects here: threads, semaphores, etc.
+	
+	// ...
+	
+	
+	// start the operating system, run the highest
+	// priority thread
+	os_start();
+	
+	// reaches the point of no return. The execution 
+	// should never get here. If it does, something 
+	// went wrong.
+	
+	for( ; ; )
+		;
+
+	return 0;
+}
+
+```