Update README

configs/stm32f103-minimum/README.txt

@@ -428,6 +428,49 @@ Where <subdir> is one of the following:
     configuration.  This configuration has far fewer features than the nsh
     configuration but is also a fraction of the size.
 
+    This minnsh configuration is a "proof-of-concept" and not very usable in
+    its current state.  This configuration was created by disabling
+    everything possible INCLUDING file system support.  Without file system
+    support, NuttX is pretty much crippled.  Here are some of the
+    consequences of disabling the file system:
+
+    - All features that depend on the file system are lost:  device drivers,
+      mountpoints, message queues, named semaphores.
+
+    - Without device drivers, you cannot interact with the RTOS using POSIX
+      interfaces.  You would have to work with NuttX as with those other
+      tiny RTOSs:  As a scheduler and a callable hardare abstraction layer
+      (HAL).
+
+    - You cannot use any of the NuttX upper half device drivers since they
+      depend on the pseudo-file system and device nodes.  You can, of
+      course, continue to use the lower half drivers either directly.  Or,
+      perhaps, you could write some custom minnsh upper half drivers that
+      do not depend on a file system and expose a HAL interface.
+
+    There is a special version of readline() the NSH uses when there is no
+    file system.  It uses a special up_putc() to write data to the console
+    and a special function up_getc() to read data from the console.
+
+    - The current up_getc() implementationsa are a kludge.  They are
+      analogous to the up_putc() implementations:  They directly poll the
+      hardware for serial availability, locking up all lower priority tasks
+      in the entire system while they poll.  So a version of NSH that uses
+      up_getc() essentially blocks the system until a character is received.
+
+    This, of course, could be fixed by creating a special, upper half
+    implementation of the interrupt-driven serial lower half (like
+    stm32_serial) that just supports single character console I/O
+    (perhaps called up_putc and up_getc?).  The NSH could wait for serial
+    input without blocking the system.  But then that would increase the
+    footprint too.
+
+    So although the minnsh configurations are a good starting point for
+    making things small, they not are really very practical.  Why might
+    you want a NuttX minnsh solution?  Perhaps you have software that runs
+    on a family of chips including some very tiny MCUs.  Then perhaps having
+    the RTOS compatibility would justify the loss of functionality?
+
     STATUS:
     2016-06-03:  Using that config I got this:
 
@@ -446,6 +489,50 @@ Where <subdir> is one of the following:
         Mem: 18624 2328 16296 16296
        nsh>
 
+    2016-06-07:  As another experiment, I tried enabling just (1) the file
+    system, (2) the console device, and (3) the upper half serial driver in
+    the minnsh configuration.  With these changes, NSH should behave better 
+    nd we preserve the device driver interface.  I made the following
+    configuration changes:
+
+      Enable the file system:
+        CONFIG_NFILE_DESCRIPTORS=5
+        CONFIG_NFILE_STREAMS=5
+
+      Enable the console device:
+        CONFIG_DEV_CONSOLE=y
+
+      Disable most new NSH commands.  Some like 'ls' are really mandatory
+      with a file system:
+        CONFIG_NSH_DISABLE_xxx=y
+
+      Enable the upper half serial driver:
+        CONFIG_SERIAL=y
+        CONFIG_STANDARD_SERIAL=y
+
+      Enable the USART1 serial driver:
+        CONFIG_STM32_USART1=y
+        CONFIG_STM32_USART1_SERIALDRIVER=y
+        CONFIG_USART1_SERIAL_CONSOLE=y
+
+        CONFIG_USART1_2STOP=0
+        CONFIG_USART1_BAUD=115200
+        CONFIG_USART1_BITS=8
+        CONFIG_USART1_PARITY=0
+        CONFIG_USART1_RXBUFSIZE=16
+        CONFIG_USART1_TXBUFSIZE=16
+
+    The resulting code was bigger as expected:
+
+      $ arm-none-eabi-size nuttx
+         text    data     bss     dec     hex filename
+        20093      88     876   21057    5241 nuttx
+
+    I am sure that other things that could be disabled were also drawn into
+    the build, so perhaps this could be reduced.  But as a ballpark
+    estimate, it looks like the cost of basic file system and serial console
+    driver support is around 7+KB.
+
   nsh:
   ---
     Configures the NuttShell (nsh) located at apps/examples/nsh. This