From b54e12eaa8cbe3ca36e4382245058de5eef0e0d1 Mon Sep 17 00:00:00 2001 From: patacongo Date: Fri, 30 Jul 2010 21:51:10 +0000 Subject: Update sim documentation git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@2819 42af7a65-404d-4744-a932-0658087f49c3 --- nuttx/configs/sim/README.txt | 75 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) (limited to 'nuttx/configs/sim') diff --git a/nuttx/configs/sim/README.txt b/nuttx/configs/sim/README.txt index 54485872b..235f42c0e 100644 --- a/nuttx/configs/sim/README.txt +++ b/nuttx/configs/sim/README.txt @@ -1,6 +1,81 @@ README ^^^^^^ +This README file describes the contents of the build configurations available +for the NuttX "sim" target. The sim target is a NuttX port that runs as a +user-space program under Linux or Cygwin. It is a very "low fidelity" embedded +system simulation: This environment does not support any kind of asynchonous +events -- there are nothing like interrupts in this context. Therefore, there +can be no pre-empting events. + +In order to get timed behavior, the system timer "interrupt handler" is called +from the sim target's IDLE loop. The IDLE runs whenever there is no other task +running. So, for example, if a task calls sleep(), then that task will suspend +wanting for the time to elapse. If nothing else is available to run, then the +IDLE loop runs and the timer increments, eventually re-awakening the sleeping task. + +Context switching is based on logic similar to setjmp() and longjmp(). + +The sim target is used primarily as a development and test platform for new +RTOS features. It is also of academic interest. But it has no real-world +application that I know of. + +NOTE: In order to facility fast testing, the sim target's IDLE loop, by default, +calls the system "interrupt handler" as fast as possible. As a result, there +really are no noticeable delays when a task sleeps. However, the task really does +sleep -- but the time scale is wrong. If you want behavior that is closer to +normal timing, then you can define CONFIG_SIM_WALLTIME=y in your configuration +file. This configuration setting will cause the sim target's IDLE loop to delay +on each call so that the system "timer interrupt" is called at a rate approximately +correct for the system timer tick rate. With this definition in the configuration, +sleep() behavior is more or less normal. + +64-Bit Issues +^^^^^^^^^^^^^ + +As mentioned above, context switching is based on logic like setjmp and longjmp. +This context switching is only available for 32-bit targets. On 64-bit machines, +this context switching will fail. + +There are other 64-bit issues as well. For example, addresses are retained in +32-bit unsigned integer types in a few places. On a 64-bit machine, the 32-bit +address storage may correcupt 64-bit addressing. NOTE: This is really a bug -- +addresses should not be retained in uint32_t types but rather in uintptr_t types +to avoid issues just like this. + +The workaround on 64-bit machines for now is to build for a 32-bit target on the +64-bit machine. This workaround involves modifying the Make.defs file in the +appropriate places so that -m32 is included in the CFLAGS and -m32 and -melf_386 +are included in the LDFLAGS. See the patch 0001-Quick-hacks-to-build-sim-nsh-ostest-on-x86_64-as-32-.patch +that can be found at http://tech.groups.yahoo.com/group/nuttx/files. + +Buffered I/O Issues +^^^^^^^^^^^^^^^^^^^ + +The simulated serial driver has some odd behavior. It will stall for a long time +on reads when the C stdio buffers are being refilled. This only effects the behavior +of things like fgetc(). Workaround: Set CONFIG_STDIO_BUFFER_SIZE=0, suppressing +all C buffered I/O. + +Networking Issues +^^^^^^^^^^^^^^^^^ + +I never did get networking to work on the sim target. It tries to use the tap device +(/dev/net/tun) to emulate an Ethernet NIC, but I never got it correctly integrated +with the NuttX networking (I probably should try using raw sockets instead). + +X11 Issues +^^^^^^^^^^ + +There is an X11-based framebuffer driver that you can use exercise the NuttX graphics +subsystem on the simulator (see the sim/nx configuration below). But I am not much +of an X11 programmer so I did not use X11 autoconfiguration stuff. As a result, the X11 +support builds on old X11 installations, but not on current versions. This needs to +get fixed someday. + +Configurations +^^^^^^^^^^^^^^ + mount Configures to use examples/mount. This configuration may be selected as follows: -- cgit v1.2.3