diff --git a/src/interrupts.s b/src/interrupts.s
index 693ca6c..81a87fe 100644
--- a/src/interrupts.s
+++ b/src/interrupts.s
@@ -369,21 +369,85 @@ fs_init:
 fs_ioctl:
 	rts
 
+; -----------------------------------------------------------------------------
+; Read a number of raw sectors from the SD card. Arguments are passed in a data
+; structure with the following layout:
+;
+; struct sector_read_s {
+;		uint32_t offset;
+;		uint16_t count;
+;		void *dest;
+; };
+;
+; Arguments:
+;		A: Bank of argument
+;		Y: Address of argument
+; Return value:
+;		A: Error code
+;		X: Number of sectors actually read
 fs_sector_read:
 	rts
 
+; -----------------------------------------------------------------------------
+; Write a number of raw sectors to the SD card. Arguments are passed in a data
+; structure with the following layout:
+;
+; struct sector_read_s {
+;		uint32_t offset;
+;		uint16_t count;
+;		void *src;
+; };
+;
+; Arguments:
+;		A: Bank of argument
+;		Y: Address of argument
+; Return value:
+;		A: Error code
+;		X: Number of sectors actually written
 fs_sector_write:
 	rts
 
+; -----------------------------------------------------------------------------
+; Format the inserted SD card, removing any existing data.
+;
+; Arguments:
+;		A: Bank of volume label string
+;		Y: Address of volume label string
+; Return value:
+;		A: Error code
 fs_mkfs:
 	rts
 
+; -----------------------------------------------------------------------------
+; Get the amount of free space (in bytes) on the filesystem.
+;
+; Arguments: None
+; Return value:
+;		X: High-order 32 bits of free space
+;		Y: Low-order 32 bits of free space
 fs_free:
 	rts
 
+; -----------------------------------------------------------------------------
+; Get the volume label of the SD card. The string is written to a buffer owned
+; by the caller.
+;
+; Arguments:
+;		A: Bank of volume label string
+;		Y: Address of volume label string
+; Return value:
+;		A: Error code
 fs_get_label:
 	rts
 
+; -----------------------------------------------------------------------------
+; Set the volume label of the SD card.
+;
+; Arguments:
+;		A: Bank of volume label string
+;		Y: Address of volume label string
+; Return value:
+;		A: Error code
 fs_set_label:
 	rts