# ARM9 No Task Switching **URL:** \ **Category:** Kernel\ **Created:** [September 4, 2011, 6:18pm UTC](https://forums.freertos.org/t/arm9-no-task-switching/4241 "2011-09-04T18:18:28Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\ **Post date:** [September 4, 2011, 6:18pm UTC](https://forums.freertos.org/t/arm9-no-task-switching/4241/1 "2011-09-04T18:18:28Z")
**[imkaush](https://sourceforge.net/u/imkaush/)** wrote on [Sunday, September 04, 2011](https://sourceforge.net/p/freertos/discussion/382005/thread/bd3ffad6/#8767): Hi all, I am trying to port FreeRTOS on Samsung S3C2440 (ARM9) processor (FriendlyArm’s Mini2440 board) using ADS 1.2 IDE I am able to start the scheduler but no task switching takes place. If I configure AppTickHook enabled from FreeRTOSConfig.h, then it is the only task being executed. If AppTickHook is disabled then only the demo task is executed…it does not suspend (using vTaskSuspend() api) nor does it get delayed (using VTaskDelay() api). I am using RVDS/ARM7\_LPC21xx port files as reference This is my assembly code for the SWI handler(in startup file), vPortYield and vPortYieldProcessor(both in portASM.s) which i think is the problem: in startup.s: ``` MACRO $HandlerLabel HANDLER $HandleLabel $HandlerLabel sub sp,sp,#4 ;decrement sp(to store jump address) stmfd sp!,{r0} ;PUSH the work register to stack(lr does''t push because it return to original address) ldr r0,=$HandleLabel ;load the address of HandleXXX to r0 ldr r0,[r0] ;load the contents(service routine start address) of HandleXXX str r0,[sp,#4] ;store the contents(ISR) of HandleXXX to stack ldmfd sp!,{r0,pc} ;POP the work register and pc(jump to ISR) MEND .... b HandlerSWI .... HandlerSWI HANDLER HandleSWI .... ; Setup SWI handler ldr r0,=HandleSWI ldr r1,=vPortYieldProcessor str r1,[r0] .... ``` In portASM.s: ``` .... vPortYield PRESERVE8 SWI 0 .... vPortYieldProcessor PRESERVE8 ; Within an IRQ ISR the link register has an offset from the true return ; address, but an SWI ISR does not. Add the offset manually so the same ; ISR return code can be used in both cases. ADD LR, LR, #4 ; Perform the context switch. portSAVE_CONTEXT ; Save current task context LDR R0, =vTaskSwitchContext ; Get the address of the context switch function MOV LR, PC ; Store the return address BX R0 ; Call the contedxt switch function portRESTORE_CONTEXT ; restore the context of the selected task ``` Thanks in advance for any help!!! ---
**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\ **Post date:** [September 5, 2011, 7:15am UTC](https://forums.freertos.org/t/arm9-no-task-switching/4241/2 "2011-09-05T07:15:13Z")
**[davedoors](https://sourceforge.net/u/davedoors/)** wrote on [Monday, September 05, 2011](https://sourceforge.net/p/freertos/discussion/382005/thread/bd3ffad6/#8d06): I don’t know how the interrupt controller works on your chip, but for chips with a vectored controller I would expect the vector table to look something like the code below. Note this is taken from an NXP example, so the IRQ interrupt will be different on your chip. The thing I notice about your code is the SWI handler is that it is not branching directly to FreeRTOS but branching indirectly through R0. I don’t know if this needs to work this way on your chip, but expect that will stop it working unless all interrupts do the exact same thing. See how it is different below. ``` Vectors LDR PC, Reset_Addr LDR PC, Undef_Addr LDR PC, SWI_Addr LDR PC, PAbt_Addr LDR PC, DAbt_Addr NOP ; Reserved Vector LDR PC, [PC, #-0x0FF0] ; Vector from VicVectAddr LDR PC, FIQ_Addr Reset_Addr DCD Reset_Handler Undef_Addr DCD Undef_Handler SWI_Addr DCD vPortYieldProcessor PAbt_Addr DCD PAbt_Handler DAbt_Addr DCD DAbt_Handler DCD 0 ; Reserved Address IRQ_Addr DCD IRQ_Handler FIQ_Addr DCD FIQ_Handler ``` Maybe your chip has a completely different interrupt flow though. In any case, I suggest setting configUSE\_PREEMPTION to 0 first, then have two tasks that yield to each other. Only when that is working add in the tick ISR. Then when that is working change configUSE\_PREEMPTION to 1 and see if it still works. ---
**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\ **Post date:** [September 5, 2011, 11:03am UTC](https://forums.freertos.org/t/arm9-no-task-switching/4241/3 "2011-09-05T11:03:45Z")
**[imkaush](https://sourceforge.net/u/imkaush/)** wrote on [Monday, September 05, 2011](https://sourceforge.net/p/freertos/discussion/382005/thread/bd3ffad6/#8040): I have already tried this: ``` HandleSWI DCD vPortYieldProcessor ``` In this case, the task is executed once and the program is restarted. I will try with configUSE\_PREEMPTION 0/1 as you suggested and get back…… ---
**Author:** ![system](https://d2v6vdsk2p900z.cloudfront.net/original/1X/7fc8fdb729c645aab5b20282bbe34e57cb407be6.jpeg) [@system](https://forums.freertos.org/u/system)\ **Post date:** [September 5, 2011, 4:32pm UTC](https://forums.freertos.org/t/arm9-no-task-switching/4241/4 "2011-09-05T16:32:07Z")
**[imkaush](https://sourceforge.net/u/imkaush/)** wrote on [Monday, September 05, 2011](https://sourceforge.net/p/freertos/discussion/382005/thread/bd3ffad6/#f116): Yes!!! It’s working….here are the changes: In startup.s : ``` //replaced... b HandlerSWI //with.... LDR PC, HandlerSWI //replaced... HandlerSWI HANDLER HandleSWI //with.... SWI_Addr DCD vPortYieldProcessor //removed... ; Setup SWI handler ldr r0,=HandleSWI ldr r1,=vPortYieldProcessor str r1,[r0] ``` In portASM.s: ``` //added "bx lr" to portYeild vPortYield PRESERVE8 SWI 0 bx lr ``` Thanks a lot davedoors!!!! Kudos!