diff --git a/os/hal/lib/complex/buffered_sio/hal_buffered_sio.c b/os/hal/lib/complex/buffered_sio/hal_buffered_sio.c
index e4524da17..509430512 100644
--- a/os/hal/lib/complex/buffered_sio/hal_buffered_sio.c
+++ b/os/hal/lib/complex/buffered_sio/hal_buffered_sio.c
@@ -75,7 +75,7 @@ static void __bsio_default_cb(SIODriver *siop) {
 
   /* Posting the non-data SIO events as channel event flags, the masks are
      made to match.*/
-  events = sioGetAndClearEventsI(siop);
+  events = sioGetAndClearEventsX(siop);
   chnAddFlagsI(bsiop, (eventflags_t)(events & ~SIO_EV_ALL_DATA));
 
   /* RX FIFO event.*/
@@ -207,7 +207,7 @@ msg_t bsioStart(BufferedSIODriver *bsiop, const BufferedSIOConfig *config) {
   if (msg == HAL_RET_SUCCESS) {
     osalSysLock();
     sioSetCallbackX(bsiop->siop, &__bsio_default_cb);
-    sioWriteEnableFlagsI(bsiop->siop, SIO_EV_ALL_EVENTS);
+    sioWriteEnableFlagsX(bsiop->siop, SIO_EV_ALL_EVENTS);
     bsiop->state = BS_READY;
     osalSysUnlock();
   }
diff --git a/os/hal/ports/sandbox/hal_sio_lld.c b/os/hal/ports/sandbox/hal_sio_lld.c
index 954d88100..e0883422a 100644
--- a/os/hal/ports/sandbox/hal_sio_lld.c
+++ b/os/hal/ports/sandbox/hal_sio_lld.c
@@ -66,14 +66,14 @@ static const SIOConfig default_config = {
 __attribute__((always_inline))
 static inline uint32_t __sio_vuart_init(uint32_t nvuart, uint32_t ncfg) {
 
-  __syscall3r(201, SB_VUART_INIT, nvuart, ncfg);
+  __syscall3r(96, SB_VUART_INIT, nvuart, ncfg);
   return (uint32_t)r0;
 }
 
 __attribute__((always_inline))
 static inline uint32_t __sio_vuart_deinit(uint32_t nvuart) {
 
-  __syscall2r(201, SB_VUART_INIT, nvuart);
+  __syscall2r(96, SB_VUART_DEINIT, nvuart);
   return (uint32_t)r0;
 }
 
@@ -217,10 +217,10 @@ void sio_lld_stop(SIODriver *siop) {
  */
 bool sio_lld_is_rx_empty(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_ISRXE, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_ISRXE, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (bool)r1;
+  return (bool)r0;
 }
 
 /**
@@ -235,10 +235,10 @@ bool sio_lld_is_rx_empty(SIODriver *siop) {
  */
 bool sio_lld_is_rx_idle(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_ISRXI, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_ISRXI, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (bool)r1;
+  return (bool)r0;
 }
 
 /**
@@ -255,10 +255,10 @@ bool sio_lld_is_rx_idle(SIODriver *siop) {
  */
 bool sio_lld_has_rx_errors(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_HASERR, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_HASERR, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (bool)r1;
+  return (bool)r0;
 }
 
 /**
@@ -273,10 +273,10 @@ bool sio_lld_has_rx_errors(SIODriver *siop) {
  */
 bool sio_lld_is_tx_full(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_ISTXF, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_ISTXF, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (bool)r1;
+  return (bool)r0;
 }
 
 /**
@@ -291,10 +291,10 @@ bool sio_lld_is_tx_full(SIODriver *siop) {
  */
 bool sio_lld_is_tx_ongoing(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_ISTXO, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_ISTXO, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (bool)r1;
+  return (bool)r0;
 }
 
 /**
@@ -304,8 +304,8 @@ bool sio_lld_is_tx_ongoing(SIODriver *siop) {
  */
 void sio_lld_update_enable_flags(SIODriver *siop) {
 
-  __syscall3r(201, SB_VUART_WREN, siop->nvuart, siop->enabled);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall3r(225, SB_VUART_WREN, siop->nvuart, siop->enabled);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 }
 
 /**
@@ -318,10 +318,10 @@ void sio_lld_update_enable_flags(SIODriver *siop) {
  */
 sioevents_t sio_lld_get_and_clear_errors(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_GCERR, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_GCERR, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (sioevents_t)r1;
+  return (sioevents_t)r0;
 }
 
 /**
@@ -334,10 +334,10 @@ sioevents_t sio_lld_get_and_clear_errors(SIODriver *siop) {
  */
 sioevents_t sio_lld_get_and_clear_events(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_GCEVT, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_GCEVT, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (sioevents_t)r1;
+  return (sioevents_t)r0;
 }
 
 /**
@@ -350,10 +350,10 @@ sioevents_t sio_lld_get_and_clear_events(SIODriver *siop) {
  */
 sioevents_t sio_lld_get_events(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_GEVT, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_GEVT, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (sioevents_t)r1;
+  return (sioevents_t)r0;
 }
 
 /**
@@ -369,10 +369,10 @@ sioevents_t sio_lld_get_events(SIODriver *siop) {
  */
 size_t sio_lld_read(SIODriver *siop, uint8_t *buffer, size_t n) {
 
-  __syscall4rr(201, SB_VUART_READ, siop->nvuart, buffer, n);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall4r(225, SB_VUART_READ, siop->nvuart, buffer, n);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (size_t)r1;
+  return (size_t)r0;
 }
 
 /**
@@ -388,10 +388,10 @@ size_t sio_lld_read(SIODriver *siop, uint8_t *buffer, size_t n) {
  */
 size_t sio_lld_write(SIODriver *siop, const uint8_t *buffer, size_t n) {
 
-  __syscall4rr(201, SB_VUART_WRITE, siop->nvuart, buffer, n);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall4r(225, SB_VUART_WRITE, siop->nvuart, buffer, n);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (size_t)r1;
+  return (size_t)r0;
 }
 
 /**
@@ -405,10 +405,10 @@ size_t sio_lld_write(SIODriver *siop, const uint8_t *buffer, size_t n) {
  */
 msg_t sio_lld_get(SIODriver *siop) {
 
-  __syscall2rr(201, SB_VUART_GET, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2r(225, SB_VUART_GET, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (size_t)r1;
+  return (size_t)r0;
 }
 
 /**
@@ -422,8 +422,8 @@ msg_t sio_lld_get(SIODriver *siop) {
  */
 void sio_lld_put(SIODriver *siop, uint_fast16_t data) {
 
-  __syscall3r(201, SB_VUART_PUT, siop->nvuart, data);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall3r(225, SB_VUART_PUT, siop->nvuart, data);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 }
 
 /**
@@ -442,10 +442,10 @@ void sio_lld_put(SIODriver *siop, uint_fast16_t data) {
  */
 msg_t sio_lld_control(SIODriver *siop, unsigned int operation, void *arg) {
 
-  __syscall4rr(201, SB_VUART_CTL, siop->nvuart, operation, arg);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall4r(225, SB_VUART_CTL, siop->nvuart, operation, arg);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
-  return (size_t)r1;
+  return (msg_t)r0;
 }
 
 /**
@@ -459,11 +459,11 @@ void sio_lld_serve_interrupt(SIODriver *siop) {
   sioevents_t events;
 
 #if SIO_USE_SYNCHRONIZATION == TRUE
-  __syscall2rr(201, SB_VUART_GEVT, siop->nvuart);
-  osalDbgAssert((msg_t)r0 == HAL_RET_SUCCESS, "unexpected failure");
+  __syscall2rr(225, SB_VUART_GEVT, siop->nvuart);
+  osalDbgAssert(r0 != (uint32_t)-1, "unexpected failure");
 
   /* Processing enabled events, if any.*/
-  events = (sioevents_t)r1 & siop->enabled;
+  events = (sioevents_t)r0 & siop->enabled;
   if (events != (sioevents_t)0) {
 
     /* Error events handled as a group.*/
diff --git a/os/sb/common/sbsysc.h b/os/sb/common/sbsysc.h
index 0c39192c9..ec8fa7df5 100644
--- a/os/sb/common/sbsysc.h
+++ b/os/sb/common/sbsysc.h
@@ -60,6 +60,7 @@
  * @{
  */
 #define SB_FASTC_VHAL_VGPIO     224
+#define SB_FASTC_VHAL_VUART     225
 #define SB_FASTC_VRQ_SETWT      248
 #define SB_FASTC_VRQ_CLRWT      249
 #define SB_FASTC_VRQ_SETEN      250
@@ -111,20 +112,20 @@
  */
 #define SB_VUART_INIT           0
 #define SB_VUART_DEINIT         1
-#define SB_VUART_ISRXE          2
-#define SB_VUART_ISRXI          3
-#define SB_VUART_ISTXF          4
-#define SB_VUART_ISTXO          5
-#define SB_VUART_HASERR         6
-#define SB_VUART_READ           7
-#define SB_VUART_WRITE          8
-#define SB_VUART_GET            9
-#define SB_VUART_PUT            10
-#define SB_VUART_WREN           11
-#define SB_VUART_GCERR          12
-#define SB_VUART_GCEVT          13
-#define SB_VUART_GEVT           14
-#define SB_VUART_CTL            15
+#define SB_VUART_ISRXE          0
+#define SB_VUART_ISRXI          1
+#define SB_VUART_ISTXF          2
+#define SB_VUART_ISTXO          3
+#define SB_VUART_HASERR         4
+#define SB_VUART_READ           5
+#define SB_VUART_WRITE          6
+#define SB_VUART_GET            7
+#define SB_VUART_PUT            8
+#define SB_VUART_WREN           9
+#define SB_VUART_GCERR          10
+#define SB_VUART_GCEVT          11
+#define SB_VUART_GEVT           12
+#define SB_VUART_CTL            13
 /** @} */
 
 /*===========================================================================*/
diff --git a/os/sb/vio/sbvio_uart.c b/os/sb/vio/sbvio_uart.c
index 622012c22..2a1bb5230 100644
--- a/os/sb/vio/sbvio_uart.c
+++ b/os/sb/vio/sbvio_uart.c
@@ -59,12 +59,11 @@ static void vuart_cb(SIODriver *siop) {
 /* Module exported functions.                                                */
 /*===========================================================================*/
 
-void sb_api_vio_uart(struct port_extctx *ectxp) {
+void sb_sysc_vio_uart(struct port_extctx *ectxp) {
   sb_class_t *sbp = (sb_class_t *)chThdGetSelfX()->ctx.syscall.p;
   uint32_t sub  = ectxp->r0;
   uint32_t unit = ectxp->r1;
   ectxp->r0 = (uint32_t)CH_RET_INNER_ERROR;
-  ectxp->r1 = (uint32_t)0;
   const vio_uart_unit_t *unitp;
 
   if (unit >= sbp->config->vioconf->uarts->n) {
@@ -110,34 +109,57 @@ void sb_api_vio_uart(struct port_extctx *ectxp) {
       ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
       break;
     }
+  default:
+    ectxp->r0 = (uint32_t)CH_RET_ENOSYS;
+    break;
+  }
+}
+
+void sb_fastc_vio_uart(struct port_extctx *ectxp) {
+  sb_class_t *sbp = (sb_class_t *)chThdGetSelfX()->ctx.syscall.p;
+  uint32_t sub  = ectxp->r0;
+  uint32_t unit = ectxp->r1;
+  const vio_uart_unit_t *unitp;
+
+  /* Returned value in case of error or illegal sub-code.*/
+  ectxp->r0 = (uint32_t)-1;
+
+  if (unit >= sbp->config->vioconf->uarts->n) {
+    return;
+  }
+
+  unitp = &sbp->config->vioconf->uarts->units[unit];
+
+  /* We don't want assertion or errors to be caused in host, making sure
+     all functions are called in the proper state.*/
+  if (unitp->siop->state != SIO_READY) {
+    return;
+  }
+
+  switch (sub) {
   case SB_VUART_ISRXE:
     {
-      ectxp->r1 = (uint32_t)sioIsRXEmptyX(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = (uint32_t)sioIsRXEmptyX(unitp->siop);
       break;
     }
   case SB_VUART_ISRXI:
     {
-      ectxp->r1 = (uint32_t)sioIsRXIdleX(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = (uint32_t)sioIsRXIdleX(unitp->siop);
       break;
     }
   case SB_VUART_ISTXF:
     {
-      ectxp->r1 = (uint32_t)sioIsTXFullX(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = (uint32_t)sioIsTXFullX(unitp->siop);
       break;
     }
   case SB_VUART_ISTXO:
     {
-      ectxp->r1 = (uint32_t)sioIsTXOngoingX(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = (uint32_t)sioIsTXOngoingX(unitp->siop);
       break;
     }
   case SB_VUART_HASERR:
     {
-      ectxp->r1 = (uint32_t)sioHasRXErrorsX(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = (uint32_t)sioHasRXErrorsX(unitp->siop);
       break;
     }
   case SB_VUART_READ:
@@ -145,18 +167,13 @@ void sb_api_vio_uart(struct port_extctx *ectxp) {
       uint8_t *buffer = (uint8_t *)ectxp->r2;
       size_t n = (size_t)ectxp->r3;
 
-      if (unitp->siop->state != SIO_READY) {
-        ectxp->r0 = (uint32_t)CH_RET_EIO;
-        break;
-      }
-
       if (!sb_is_valid_write_range(sbp, buffer, n)) {
-        ectxp->r0 = (uint32_t)CH_RET_EFAULT;
+        ectxp->r0 = (uint32_t)0;
+        /* TODO enforce fault instead.*/
         break;
       }
 
-      ectxp->r1 = sioAsyncRead(unitp->siop, buffer, n);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = sioAsyncReadX(unitp->siop, buffer, n);
       break;
     }
   case SB_VUART_WRITE:
@@ -164,94 +181,51 @@ void sb_api_vio_uart(struct port_extctx *ectxp) {
       const uint8_t *buffer = (const uint8_t *)ectxp->r2;
       size_t n = (size_t)ectxp->r3;
 
-      if (unitp->siop->state != SIO_READY) {
-        ectxp->r0 = (uint32_t)CH_RET_EIO;
-        break;
-      }
-
       if (!sb_is_valid_read_range(sbp, buffer, n)) {
-        ectxp->r0 = (uint32_t)CH_RET_EFAULT;
+        ectxp->r0 = (uint32_t)0;
+        /* TODO enforce fault instead.*/
         break;
       }
 
-      ectxp->r1 = sioAsyncWrite(unitp->siop, buffer, n);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = sioAsyncWriteX(unitp->siop, buffer, n);
       break;
     }
   case SB_VUART_GET:
     {
-      if (unitp->siop->state != SIO_READY) {
-        ectxp->r0 = (uint32_t)CH_RET_EIO;
-        break;
-      }
-
-      ectxp->r1 = sioGetX(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = sioGetX(unitp->siop);
       break;
     }
   case SB_VUART_PUT:
     {
-      uint_fast16_t data = (uint_fast16_t)ectxp->r2;
-
-      if (unitp->siop->state != SIO_READY) {
-        ectxp->r0 = (uint32_t)CH_RET_EIO;
-        break;
-      }
-
-      sioPutX(unitp->siop, data);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      sioPutX(unitp->siop, (uint_fast16_t)ectxp->r2);
+      ectxp->r0 = (uint32_t)0;
       break;
     }
   case SB_VUART_WREN:
     {
-      sioevents_t flags = (sioevents_t)ectxp->r2;
-
-      if (unitp->siop->state != SIO_READY) {
-        ectxp->r0 = (uint32_t)CH_RET_EIO;
-        break;
-      }
-
-      sioWriteEnableFlags(unitp->siop, flags);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      sioWriteEnableFlagsX(unitp->siop, (sioevents_t)ectxp->r2);
+      ectxp->r0 = (uint32_t)0;
       break;
     }
   case SB_VUART_GCERR:
     {
-      if (unitp->siop->state != SIO_READY) {
-        ectxp->r0 = (uint32_t)CH_RET_EIO;
-        break;
-      }
-
-      ectxp->r1 = (uint32_t)sioGetAndClearErrors(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = (uint32_t)sioGetAndClearErrorsX(unitp->siop);
       break;
     }
   case SB_VUART_GCEVT:
     {
-      if (unitp->siop->state != SIO_READY) {
-        ectxp->r0 = (uint32_t)CH_RET_EIO;
-        break;
-      }
-
-      ectxp->r1 = (uint32_t)sioGetAndClearEvents(unitp->siop);
-      ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
+      ectxp->r0 = (uint32_t)sioGetAndClearEventsX(unitp->siop);
       break;
     }
   case SB_VUART_GEVT:
-  {
-    if (unitp->siop->state != SIO_READY) {
-      ectxp->r0 = (uint32_t)CH_RET_EIO;
+    {
+      ectxp->r0 = (uint32_t)sioGetEventsX(unitp->siop);
       break;
     }
-
-    ectxp->r1 = (uint32_t)sioGetEvents(unitp->siop);
-    ectxp->r0 = (uint32_t)HAL_RET_SUCCESS;
-    break;
-  }
   case SB_VUART_CTL:
     /* falls through */
   default:
-    ectxp->r0 = (uint32_t)CH_RET_ENOSYS;
+    /* Silently ignored.*/
     break;
   }
 }
diff --git a/os/sb/vio/sbvio_uart.h b/os/sb/vio/sbvio_uart.h
index 616b531bd..2f307b546 100644
--- a/os/sb/vio/sbvio_uart.h
+++ b/os/sb/vio/sbvio_uart.h
@@ -37,7 +37,12 @@
 /**
  * @brief   API handler for VHAL PAL driver.
  */
-#define SB_SVC201_HANDLER       sb_api_vio_uart
+#define SB_SVC96_HANDLER        sb_sysc_vio_uart
+
+/**
+ * @brief   Fast API handler for VHAL PAL driver.
+ */
+#define SB_SVC225_HANDLER       sb_fastc_vio_uart
 
 /*===========================================================================*/
 /* Module pre-compile time settings.                                         */
@@ -94,7 +99,8 @@ typedef struct vio_uart_configs {
 #ifdef __cplusplus
 extern "C" {
 #endif
-  void sb_api_vio_uart(struct port_extctx *ectxp);
+void sb_sysc_vio_uart(struct port_extctx *ectxp);
+void sb_fastc_vio_uart(struct port_extctx *ectxp);
 #ifdef __cplusplus
 }
 #endif