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+# agon-mos API
+
+The API can be used by external applications to access MOS functionality
+
+## Usage from Z80 assembler
+
+There are three RST instructions for accessing MOS functionality from Z80.
+
+`RST 00h`: Reset the eZ80
+`RST 08h`: Execute a MOS command
+`RST 10h`: Output a character to the VDP
+
+## Examples
+
+Include the file mos_api.inc in your project.
+As the MOS code is in the eZ80 FLASH ROM from address zero, care must be takn when running code in compatibility mode within a 64K segment.
+For example, BBC BASIC for AGON uses the following code for outputting a character to the VDP
+
+```
+; OSWRCH: Write a character out to the ESP32 VDU handler via the MOS
+; A: Character to write
+;
+OSWRCH:		RST.LIS	10h			; This calls a RST in the eZ80 address space
+			RET
+```
+
+The `RST.LIS` ensures the MOS RST instructions are called regardless of the eZ80s current addressing mode
+
+There is a macro in mos_api.inc for calling MOS commands - there is one parameter, the mos command number
+
+```
+; OSRDCH: Read a character in from the ESP32 keyboard handler
+;
+OSRDCH:		MOSCALL	mos_getkey
+			OR	A 		
+			JR	Z, OSRDCH		; Loop until key is pressed
+			RET
+```
+
+## MOS commands
+
+MOS commands can be executed from a classic 64K Z80 segment or whilst the eZ80 is running in 24-bit ADL mode. For classic mode, 16 bit registers are passed as pointers to the MOS commands; these are automatically promoted to 24 bit by adding the MB register to bits 16-23 of the register. When running in ADL mode, a 24-bit register will be passed, but MB must be set to 0.
+
+See mos_api.asm for implementation
+
+The following MOS commands are supported
+
+### mos_getkey: Read a keypress from the VDP
+
+Parameters: None
+
+Returns:
+- A: The keycode of the character pressed
+
+### mos_load: Load a file from SD card
+
+Parameters: 
+- HL(U): Address of filename (zero terminated)
+- DE(U): Address at which to load
+- BC(U): Maximum allowed size (bytes)
+
+Returns:
+- A: File error, or 0 if OK
+- F: Carry reset if no room for file, otherwise set
+
+### mos_save: Save a file to SD card
+
+Parameters: 
+
+- HL(U): Address of filename (zero terminated)
+- DE(U): Address to save from
+- BC(U): Number of bytes to save
+
+Returns:
+
+- A: File error, or 0 if OK
+- F: Carry set
+
+### mos_cd: Change current directory on the SD card
+
+Parameters: 
+
+- HL(U): Address of path (zero terminated)
+
+Returns:
+
+- A: File error, or 0 if OK
+
+### mos_dir: List SD card folder contents
+
+Parameters: None
+
+Returns:
+
+- A: File error, or 0 if OK
+
+### mos_del: Delete a file or folder from the SD card
+
+Parameters: 
+
+- HL(U): Address of path (zero terminated)
+
+Returns:
+
+- A: File error, or 0 if OK
+
+### mos_ren: Rename a file on the SD card
+
+Parameters: 
+
+- HL(U): Address of filename1 (zero terminated)
+- D(E)U: Address of filename2 (zero terminated)
+
+Returns:
+
+- A: File error, or 0 if OK
+
+### mos_mkdir: Make a folder on the SD card
+
+Parameters: 
+
+- HL(U): Address of path (zero terminated)
+
+Returns:
+
+- A: File error, or 0 if OK
+
+### mos_sysvars: Fetch a pointer to the system variables
+
+Parameters: None
+
+Returns:
+
+- IXU: Pointer to the MOS system variable area (this is always 24 bit)
+
+### mos_editline: Invoke the line editor
+
+Parameters: 
+
+- HL(U): Address of the buffer
+- BC(U): Buffer length
+- E: 0 to not clear buffer, 1 to clear
+
+Returns:
+- A: Key that was used to exit the input loop (CR=13, ESC=27)
+
+### mos_fopen: Get a file handle
+
+Parameters: 
+
+- HL(U): Address of filename (zero terminated)
+- C: Mode
+
+Returns:
+- A: Filehandle, or 0 if couldn't open
+
+Mode can be one of: fa_read, fa_write, fa_open_existing, fa_create_new, fa_create_always, fa_open_always or fa_open_append
+
+### mos_fclose: Close a file handle
+
+Parameters: 
+
+- C: Filehandle, or 0 to close all open files
+
+Returns:
+
+- A: Number of files still open
+
+### mos_fgetc: Get a character from an open file
+
+Parameters: 
+
+- C: Filehandle
+
+Returns:
+- A: Character read
+- F: C set if last character in file, otherwise NC
+
+### mos_fputc: Write a character to an open file
+
+Parameters: 
+
+- C: Filehandle
+- B: Character to write
+
+Returns: None
+
+### mos_feof: Check for end of file
+
+Parameters: 
+
+- C: Filehandle
+
+Returns:
+- A: 1 if at end of file, otherwise 0
+
+### mos_getError: Copy an error string to a buffer
+
+Parameters: 
+
+- E: The error code
+- HL(U): Address of buffer to copy message into
+- BC(U): Size of buffer
+
+Returns: None
+