Interrupt priority assert improvements for CM3/4/7

In the ARM_CM3, ARM_CM4, and ARM_CM7 ports, change the assertion that `configMAX_SYSCALL_INTERRUPT_PRIORITY` is nonzero to account for the number of priority bits implemented by the hardware. Change these ports to also use the lowest priority for PendSV and SysTick, ignoring `configKERNEL_INTERRUPT_PRIORITY`.
FreeRTOS · Dec 17, 2022 · 0675381 · 0675381
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commit 0675381
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Showing 16 changed files with 176 additions and 88 deletions.
diff --git a/portable/CCS/ARM_CM3/port.c b/portable/CCS/ARM_CM3/port.c
@@ -52,8 +52,9 @@
 #define portNVIC_PEND_SYSTICK_SET_BIT         ( 1UL << 26UL )
 #define portNVIC_PEND_SYSTICK_CLEAR_BIT       ( 1UL << 25UL )
 
-#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY            ( 255UL )
+#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
 #define portFIRST_USER_INTERRUPT_NUMBER       ( 16 )
@@ -239,6 +240,14 @@ BaseType_t xPortStartScheduler( void )
         /* Use the same mask on the maximum system call priority. */
         ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+        /* Check that the maximum system call priority is nonzero after
+         * accounting for the number of priority bits supported by the
+         * hardware. A priority of 0 is invalid because setting the BASEPRI
+         * register to 0 unmasks all interrupts, and interrupts with priority 0
+         * cannot be masked using BASEPRI.
+         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+        configASSERT( ucMaxSysCallPriority );
+
         /* Calculate the maximum acceptable priority group value for the number
          * of bits read back. */
         ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/CCS/ARM_CM4F/port.c b/portable/CCS/ARM_CM4F/port.c
@@ -56,8 +56,9 @@
 #define portNVIC_PEND_SYSTICK_SET_BIT         ( 1UL << 26UL )
 #define portNVIC_PEND_SYSTICK_CLEAR_BIT       ( 1UL << 25UL )
 
-#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY            ( 255UL )
+#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
 #define portFIRST_USER_INTERRUPT_NUMBER       ( 16 )
@@ -258,6 +259,14 @@ BaseType_t xPortStartScheduler( void )
         /* Use the same mask on the maximum system call priority. */
         ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+        /* Check that the maximum system call priority is nonzero after
+         * accounting for the number of priority bits supported by the
+         * hardware. A priority of 0 is invalid because setting the BASEPRI
+         * register to 0 unmasks all interrupts, and interrupts with priority 0
+         * cannot be masked using BASEPRI.
+         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+        configASSERT( ucMaxSysCallPriority );
+
         /* Calculate the maximum acceptable priority group value for the number
          * of bits read back. */
         ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/GCC/ARM_CM3/port.c b/portable/GCC/ARM_CM3/port.c
@@ -55,8 +55,9 @@
 #define portNVIC_PEND_SYSTICK_SET_BIT         ( 1UL << 26UL )
 #define portNVIC_PEND_SYSTICK_CLEAR_BIT       ( 1UL << 25UL )
 
-#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY            ( 255UL )
+#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
 #define portFIRST_USER_INTERRUPT_NUMBER       ( 16 )
@@ -265,10 +266,6 @@ static void prvPortStartFirstTask( void )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
-     * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );
-
     #if ( configASSERT_DEFINED == 1 )
     {
         volatile uint32_t ulOriginalPriority;
@@ -293,6 +290,14 @@ BaseType_t xPortStartScheduler( void )
         /* Use the same mask on the maximum system call priority. */
         ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+        /* Check that the maximum system call priority is nonzero after
+         * accounting for the number of priority bits supported by the
+         * hardware. A priority of 0 is invalid because setting the BASEPRI
+         * register to 0 unmasks all interrupts, and interrupts with priority 0
+         * cannot be masked using BASEPRI.
+         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+        configASSERT( ucMaxSysCallPriority );
+
         /* Calculate the maximum acceptable priority group value for the number
          * of bits read back. */
         ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/GCC/ARM_CM3_MPU/port.c b/portable/GCC/ARM_CM3_MPU/port.c
@@ -83,8 +83,9 @@
 /* Constants required to access and manipulate the SysTick. */
 #define portNVIC_SYSTICK_INT                      ( 0x00000002UL )
 #define portNVIC_SYSTICK_ENABLE                   ( 0x00000001UL )
-#define portNVIC_PENDSV_PRI                       ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                      ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY                ( 255UL )
+#define portNVIC_PENDSV_PRI                       ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                      ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 #define portNVIC_SVC_PRI                          ( ( ( uint32_t ) configMAX_SYSCALL_INTERRUPT_PRIORITY - 1UL ) << 24UL )
 
 /* Constants required to set up the initial stack. */
@@ -381,10 +382,6 @@ static void prvRestoreContextOfFirstTask( void )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.  See
-     * https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( ( configMAX_SYSCALL_INTERRUPT_PRIORITY ) );
-
     #if ( configASSERT_DEFINED == 1 )
         {
             volatile uint32_t ulOriginalPriority;
@@ -409,6 +406,14 @@ BaseType_t xPortStartScheduler( void )
             /* Use the same mask on the maximum system call priority. */
             ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+            /* Check that the maximum system call priority is nonzero after
+             * accounting for the number of priority bits supported by the
+             * hardware. A priority of 0 is invalid because setting the BASEPRI
+             * register to 0 unmasks all interrupts, and interrupts with priority 0
+             * cannot be masked using BASEPRI.
+             * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+            configASSERT( ucMaxSysCallPriority );
+
             /* Calculate the maximum acceptable priority group value for the number
              * of bits read back. */
             ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/GCC/ARM_CM4F/port.c b/portable/GCC/ARM_CM4F/port.c
@@ -58,8 +58,9 @@
 #define portCORTEX_M7_r0p1_ID                 ( 0x410FC271UL )
 #define portCORTEX_M7_r0p0_ID                 ( 0x410FC270UL )
 
-#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY            ( 255UL )
+#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
 #define portFIRST_USER_INTERRUPT_NUMBER       ( 16 )
@@ -295,10 +296,6 @@ static void prvPortStartFirstTask( void )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
-     * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );
-
     /* This port can be used on all revisions of the Cortex-M7 core other than
      * the r0p1 parts.  r0p1 parts should use the port from the
      * /source/portable/GCC/ARM_CM7/r0p1 directory. */
@@ -329,6 +326,14 @@ BaseType_t xPortStartScheduler( void )
         /* Use the same mask on the maximum system call priority. */
         ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+        /* Check that the maximum system call priority is nonzero after
+         * accounting for the number of priority bits supported by the
+         * hardware. A priority of 0 is invalid because setting the BASEPRI
+         * register to 0 unmasks all interrupts, and interrupts with priority 0
+         * cannot be masked using BASEPRI.
+         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+        configASSERT( ucMaxSysCallPriority );
+
         /* Calculate the maximum acceptable priority group value for the number
          * of bits read back. */
         ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/GCC/ARM_CM4_MPU/port.c b/portable/GCC/ARM_CM4_MPU/port.c
@@ -93,8 +93,9 @@
 /* Constants required to access and manipulate the SysTick. */
 #define portNVIC_SYSTICK_INT                      ( 0x00000002UL )
 #define portNVIC_SYSTICK_ENABLE                   ( 0x00000001UL )
-#define portNVIC_PENDSV_PRI                       ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                      ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY                ( 255UL )
+#define portNVIC_PENDSV_PRI                       ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                      ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 #define portNVIC_SVC_PRI                          ( ( ( uint32_t ) configMAX_SYSCALL_INTERRUPT_PRIORITY - 1UL ) << 24UL )
 
 /* Constants required to manipulate the VFP. */
@@ -412,10 +413,6 @@ static void prvRestoreContextOfFirstTask( void )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.  See
-     * https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( ( configMAX_SYSCALL_INTERRUPT_PRIORITY ) );
-
     /* Errata 837070 workaround must only be enabled on Cortex-M7 r0p0
      * and r0p1 cores. */
     #if ( configENABLE_ERRATA_837070_WORKAROUND == 1 )
@@ -452,6 +449,14 @@ BaseType_t xPortStartScheduler( void )
             /* Use the same mask on the maximum system call priority. */
             ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+            /* Check that the maximum system call priority is nonzero after
+             * accounting for the number of priority bits supported by the
+             * hardware. A priority of 0 is invalid because setting the BASEPRI
+             * register to 0 unmasks all interrupts, and interrupts with priority 0
+             * cannot be masked using BASEPRI.
+             * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+            configASSERT( ucMaxSysCallPriority );
+
             /* Calculate the maximum acceptable priority group value for the number
              * of bits read back. */
             ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/GCC/ARM_CM7/r0p1/port.c b/portable/GCC/ARM_CM7/r0p1/port.c
@@ -52,8 +52,9 @@
 #define portNVIC_PEND_SYSTICK_SET_BIT         ( 1UL << 26UL )
 #define portNVIC_PEND_SYSTICK_CLEAR_BIT       ( 1UL << 25UL )
 
-#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY            ( 255UL )
+#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
 #define portFIRST_USER_INTERRUPT_NUMBER       ( 16 )
@@ -289,10 +290,6 @@ static void prvPortStartFirstTask( void )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
-     * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );
-
     #if ( configASSERT_DEFINED == 1 )
     {
         volatile uint32_t ulOriginalPriority;
@@ -317,6 +314,14 @@ BaseType_t xPortStartScheduler( void )
         /* Use the same mask on the maximum system call priority. */
         ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+        /* Check that the maximum system call priority is nonzero after
+         * accounting for the number of priority bits supported by the
+         * hardware. A priority of 0 is invalid because setting the BASEPRI
+         * register to 0 unmasks all interrupts, and interrupts with priority 0
+         * cannot be masked using BASEPRI.
+         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+        configASSERT( ucMaxSysCallPriority );
+
         /* Calculate the maximum acceptable priority group value for the number
          * of bits read back. */
         ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/IAR/ARM_CM3/port.c b/portable/IAR/ARM_CM3/port.c
@@ -55,8 +55,9 @@
 #define portNVIC_PEND_SYSTICK_SET_BIT         ( 1UL << 26UL )
 #define portNVIC_PEND_SYSTICK_CLEAR_BIT       ( 1UL << 25UL )
 
-#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY            ( 255UL )
+#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
 #define portFIRST_USER_INTERRUPT_NUMBER       ( 16 )
@@ -214,10 +215,6 @@ static void prvTaskExitError( void )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
-     * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );
-
     #if ( configASSERT_DEFINED == 1 )
     {
         volatile uint32_t ulOriginalPriority;
@@ -242,6 +239,14 @@ BaseType_t xPortStartScheduler( void )
         /* Use the same mask on the maximum system call priority. */
         ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+        /* Check that the maximum system call priority is nonzero after
+         * accounting for the number of priority bits supported by the
+         * hardware. A priority of 0 is invalid because setting the BASEPRI
+         * register to 0 unmasks all interrupts, and interrupts with priority 0
+         * cannot be masked using BASEPRI.
+         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+        configASSERT( ucMaxSysCallPriority );
+
         /* Calculate the maximum acceptable priority group value for the number
          * of bits read back. */
         ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/IAR/ARM_CM4F/port.c b/portable/IAR/ARM_CM4F/port.c
@@ -65,8 +65,9 @@
 #define portCORTEX_M7_r0p1_ID                 ( 0x410FC271UL )
 #define portCORTEX_M7_r0p0_ID                 ( 0x410FC270UL )
 
-#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY            ( 255UL )
+#define portNVIC_PENDSV_PRI                   ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                  ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
 #define portFIRST_USER_INTERRUPT_NUMBER       ( 16 )
@@ -239,10 +240,6 @@ static void prvTaskExitError( void )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
-     * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );
-
     /* This port can be used on all revisions of the Cortex-M7 core other than
      * the r0p1 parts.  r0p1 parts should use the port from the
      * /source/portable/GCC/ARM_CM7/r0p1 directory. */
@@ -273,6 +270,14 @@ BaseType_t xPortStartScheduler( void )
         /* Use the same mask on the maximum system call priority. */
         ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+        /* Check that the maximum system call priority is nonzero after
+         * accounting for the number of priority bits supported by the
+         * hardware. A priority of 0 is invalid because setting the BASEPRI
+         * register to 0 unmasks all interrupts, and interrupts with priority 0
+         * cannot be masked using BASEPRI.
+         * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+        configASSERT( ucMaxSysCallPriority );
+
         /* Calculate the maximum acceptable priority group value for the number
          * of bits read back. */
         ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;

diff --git a/portable/IAR/ARM_CM4F_MPU/port.c b/portable/IAR/ARM_CM4F_MPU/port.c
@@ -104,8 +104,9 @@
 #define portCORTEX_M7_r0p1_ID                     ( 0x410FC271UL )
 #define portCORTEX_M7_r0p0_ID                     ( 0x410FC270UL )
 
-#define portNVIC_PENDSV_PRI                       ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )
-#define portNVIC_SYSTICK_PRI                      ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )
+#define portMIN_INTERRUPT_PRIORITY                ( 255UL )
+#define portNVIC_PENDSV_PRI                       ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 16UL )
+#define portNVIC_SYSTICK_PRI                      ( ( ( uint32_t ) portMIN_INTERRUPT_PRIORITY ) << 24UL )
 #define portNVIC_SVC_PRI                          ( ( ( uint32_t ) configMAX_SYSCALL_INTERRUPT_PRIORITY - 1UL ) << 24UL )
 
 /* Constants required to check the validity of an interrupt priority. */
@@ -346,10 +347,6 @@ void vPortSVCHandler_C( uint32_t * pulParam )
  */
 BaseType_t xPortStartScheduler( void )
 {
-    /* configMAX_SYSCALL_INTERRUPT_PRIORITY must not be set to 0.
-     * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
-    configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );
-
     /* Errata 837070 workaround must only be enabled on Cortex-M7 r0p0
      * and r0p1 cores. */
     #if ( configENABLE_ERRATA_837070_WORKAROUND == 1 )
@@ -386,6 +383,14 @@ BaseType_t xPortStartScheduler( void )
             /* Use the same mask on the maximum system call priority. */
             ucMaxSysCallPriority = configMAX_SYSCALL_INTERRUPT_PRIORITY & ucMaxPriorityValue;
 
+            /* Check that the maximum system call priority is nonzero after
+             * accounting for the number of priority bits supported by the
+             * hardware. A priority of 0 is invalid because setting the BASEPRI
+             * register to 0 unmasks all interrupts, and interrupts with priority 0
+             * cannot be masked using BASEPRI.
+             * See https://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */
+            configASSERT( ucMaxSysCallPriority );
+
             /* Calculate the maximum acceptable priority group value for the number
              * of bits read back. */
             ulMaxPRIGROUPValue = portMAX_PRIGROUP_BITS;