diff --git a/README.md b/README.md
index b394707..2137319 100644
--- a/README.md
+++ b/README.md
@@ -30,12 +30,88 @@ the minimum memory requirement may be less than 64k.
 |...   |var      |Heap, growing up|
 |0x0100|var      |Transient Program Area|
 |0x0080|128 bytes|Command line and disk buffer|
-|0x0008|120 bytes|BIOS data area|
+|0x006C|20 bytes |Pre-filled FCB 2|
+|0x005C|16 bytes |Pre-filled FCB 1|
+|0x0008|84 bytes |BIOS data area, specific to BIOS implementation|
 |0x0005|3 bytes  |Jump code for BDOS|
-|0x0004|1 bytes  |default drive for BDOS|
-|0x0003|1 bytes  |may be used as iobyte by BIOS|
+|0x0004|1 byte   |default drive for BDOS|
+|0x0003|1 byte   |may be used as iobyte by BIOS|
 |0x0000|3 bytes  |Jump to BIOS warm boot entry point / program exit|
 
+## BIOS Interface
+
+The entry points for the BIOS start at its base address.
+Each entry point is offset a multiple of 3 from the beginning of the BIOS.
+
+### BIOS+0 Boot
+
+The bootloader jumps here after loading the BIOS.
+
+### BIOS+3 Warm boot
+
+Reload the BDOS from disk, and call its entry point.
+The jump opcodes at the start of memory are re-initialized.
+
+### BIOS+6 Console status
+
+Checks if keyboard input is available.
+Returns FFh in AL (A) is character is available, 00 if not.
+
+### BIOS+9 Console read
+
+Read a character from keyboard from AL (A).
+This call does not return until a character is available.
+
+### BIOS+12 Console write
+
+Write character in DL (E) to the display.
+
+### BIOS+24 Seek disk to zero
+
+Seek current disk (selected with the following call) to its beginning.
+This is a commonly a no-op.
+
+### BIOS+27 Select disk
+
+Select disk by the number given in CL (C).
+
+This is equivalent to the DL number for the PC BIOS Int 13h.
+
+### BIOS+30 Set track
+
+Select track number for next disk i/o via register CX (BC).
+
+For CHS-adressed disks, the track number is the current cylinder multiplied by the number of heads plus the current head number.
+
+For disks with one side, the cylinder number is equal to the track number.
+
+### BIOS+33 Set disk sector
+
+Select sector on the current track in register CX (BC). This parameter is 1-based, this means the first sector is sector 1. The value 0 is invalid.
+
+For CHS-adressed floppies, the number of sectors per track might vary depending on the inserted floppy disk, even for the same drive.
+
+### BIOS+36 Set disk transfer address
+
+Set buffer for disk operations via register CX (BC).
+
+This is somewhat incompatible with CP/M versions since on modern systems, the sector size is 512 instead of 128 bytes.
+Setting it to the default value of 0x80 (which is the standard disk buffer for CP/M) will overwrite the first 0x180 bytes of the transient program area.
+
+The BDOS is expected to do deblocking to provide the application program with 128-byte disk i/o.
+
+### BIOS+39 Read sector
+
+Read sector previously specified.
+AL (A) is non-zero error code on failure.
+The error codes are implementation-specific.
+
+### BIOS+42 Write sector
+
+Write sector previously specified.
+AL (A) is non-zero error code on failure.
+The error codes are implementation-specific
+
 ## Application program interface
 
 It is using the `call 5` convention with CP/M compatible syscall numbers.