diff --git a/ports/stm32f4/common-hal/busio/SPI.c b/ports/stm32f4/common-hal/busio/SPI.c
index f98d29054..b5517f470 100644
--- a/ports/stm32f4/common-hal/busio/SPI.c
+++ b/ports/stm32f4/common-hal/busio/SPI.c
@@ -38,6 +38,8 @@
 
 #define MAX_SPI 6 //TODO; replace this as part of periph cleanup
 #define ALL_CLOCKS 0xFF
+
+//arrays use 0 based numbering: SPI1 is stored at index 0
 STATIC bool reserved_spi[MAX_SPI];
 STATIC bool never_reset_spi[MAX_SPI];
 
@@ -360,4 +362,4 @@ STATIC void spi_clock_disable(uint8_t mask) {
     #ifdef SPI6
     if (mask & 1<<5) __HAL_RCC_SPI6_CLK_DISABLE();
     #endif
-}
\ No newline at end of file
+}