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--- sd:appendix_7_kernal_functions [2026/04/21 12:10] – appledog
+++ sd:appendix_7_kernal_functions [2026/04/21 12:12] (current) – appledog
@@ Line 289: / Line 289: @@
 ;
 ; ============================================================================
+</code>
+== INT 15h - Filesystem Services
+<code>
+; ============================================================================
+; INT 15h - FILE SERVICES
+; Stream-oriented file I/O for the SD-8516 / VC-3
+; ============================================================================
+;
+; Provides file and directory operations for programs.
+;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+; NOTE: INT $15 uses a JZ-style calltable.
+;       This means you cannot CALL into these functions by name.
+;       You must use INT 0x15 to call them (even from the kernal).
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;
+; Function dispatch:
+;
+; Group A: File Handle Operations ($00–$0F)
+;   AH=$00: FOPEN    - Open a file (returns handle)
+;   AH=$01: FCLOSE   - Close a file handle
+;   AH=$02: FREAD    - Read bytes from a file
+;   AH=$04: FWRITE   - Write bytes to a file
+;
+; Group B: Directory Listing ($10–$1F)
+;   AH=$12: FLIST    - List directory contents to buffer
+;
+; Group C: Directory Navigation ($20–$2F)
+;   AH=$20: FGETCWD  - Get current working directory
+;   AH=$21: FCHDIR   - Change directory
+;   AH=$22: FMKDIR   - Create directory
+;   AH=$23: FRMDIR   - Remove directory
+;
+; ============================================================================
+; --- File Command Block (FCB) ---
+; A shared memory region used to pass parameters to/from JavaScript.
+; Located in bank 0 scratch space. 16 bytes.
+;
+.equ FS_CMD_BLOCK      $00FD00     ; Base address of command block
+.equ FS_CMD            $00FD00     ; +0: Command code (1 byte)
+.equ FS_HANDLE         $00FD01     ; +1: File handle (1 byte, 0-7)
+.equ FS_MODE           $00FD02     ; +2: Mode (1 byte, for fopen: 0=read, 1=write)
+.equ FS_STATUS         $00FD03     ; +3: Result status (1 byte)
+                                   ;     $00=ok, $01=error, $02=eof
+.equ FS_COUNT          $00FD04     ; +4: Byte count (2 bytes, requested/returned)
+.equ FS_ADDR           $00FD06     ; +6: 24-bit address (buffer or filename ptr)
+.equ FS_DONE           $00FD09     ; +9: Completion flag (1 byte, 0=pending, 1=done)
+.equ FS_EXTRA          $00FD0A     ; +10: Reserved (3 bytes) and alias for:
+.equ FS_ADDR2          $00FD0A;    ;  +A: second 24-bit address (3 bytes)
+; --- Constants ---
+.equ FS_MAX_HANDLES    8           ; Maximum simultaneous open files
+.equ FS_MODE_READ      0
+.equ FS_MODE_WRITE     1
+.equ FS_OK             0
+.equ FS_ERR            1
+.equ FS_EOF            2
 </code>