The VC-3 is a system, running on the SD-8516.

I guess this interrupt system is part of the KERNAL, so should be more “VC-3” as it is system software. What follows is a simple list of the interrupt, what it does, it's calling parameters and it's return codes. I guess this should be called “Stellar BIOS V1” or “VC-3 KERNAL ROM” could be a nice name.

Each INT subsystem has it's own convention. This calling convention is not shown here; however, it will be added later and/or some functions may be explained in the SD-8516 Programmer's Reference Guide. Over time, more information will be added here.

• Function number: AH
• Small values such as char can go in AL. Otherwise, inputs can go in B.
• Pointers: ELM, ELD

• Values: B, and C if needed
• Status: Carry flag
• Pointers: ELM, FLD (see calling convention)

LDAL #'A'       ; AL has the variable name
LDB #1234       ; B has the value to set
LDAH $11        ; VAR_SET
INT $05

LDELM @string   ; address of string containing a number
LDAH $62        ; IO_GETNUM  
INT $05
; B contains the number from the string

; ============================================================================
; INT 05h - "TinyBASIC" Interpretive Language
; Complete implementation of a TinyBASIC IL + Stellar BASIC extensions
; ============================================================================
; Function dispatch via AH register:
;
; === EXECUTION CONTROL (AH=$00-$0F) ===
;   AH=$00: RUN_PROGRAM    - Start execution from line number or beginning
;   AH=$01: STOP_PROGRAM   - Stop execution
;   AH=$02: CONTINUE       - Continue from current position
;   AH=$03: EXEC_IL_STEP   - Execute one IL instruction
;   AH=$04: EXEC_IL_CONT   - Execute IL until completion
;   AH=$05: GET_STATUS     - Get interpreter status
;   AH=$06: SET_STATUS     - Set interpreter status flags
;   AH=$07: RESET_INTERP   - Reset interpreter state
;
; === VARIABLE MANAGEMENT (AH=$10-$1F) ===
;   AH=$10: VAR_GET        - Get variable value by letter
;   AH=$11: VAR_SET        - Set variable value by letter
;   AH=$12: VAR_CLEAR_ALL  - Clear all variables to 0
;   AH=$13: VAR_GET_ADDR   - Get address of variable storage
;
; === NAMED VARIABLE SERVICES (AH=$14-$1F) ===
;   AH=$14: NVAR_GET    - Find variable by name
;   AH=$15: NVAR_SET    - Insert or update variable
;   AH=$16: NVAR_DELETE - Remove variable
;   AH=$17: NVAR_CLEAR  - Clear all named variables
;
; === EXPRESSION STACK (AH=$20-$2F) ===
;   AH=$20: EXPR_PUSH      - Push value to expression stack
;   AH=$21: EXPR_POP       - Pop value from expression stack
;   AH=$22: EXPR_PEEK      - Peek at top of stack
;   AH=$23: EXPR_CLEAR     - Clear expression stack
;   AH=$24: EXPR_DEPTH     - Get current stack depth
;
; === STACK MANIPULATION (AH=$28-$2F) ===
;   AH=$25: EXPR_DUP        - Duplicate top of stack
;   AH=$26: EXPR_SWAP       - Swap top two values
;   AH=$27: EXPR_DROP       - Drop top value
;   AH=$28: EXPR_OVER       - Copy second value to top
;   AH=$29: EXPR_ROT        - Rotate top 3 values
;
; === PROGRAM LINE MANAGEMENT (AH=$30-$3F) ===
; $30-$37: Search/Navigate
;   $30: LINE_FIND          - Find line by number (forward)
;   $31: LINE_NEXT          - Get next line
;   $32: LINE_FIND_REVERSE  - Find line by number (backward)
;   $33: LINE_PREV          - Get previous line
; $36-$39: Modify
;   $36: LINE_INSERT        - Insert or replace line
;   $37: LINE_DELETE        - Delete line by number
;   $38: LINE_REMOVE_SPACE  - Add space for new program line
;   $39: LINE_MAKE_SPACE    - remove unused line space
; $3A-$3F: Utility
;   $3A: LINE_FIRST         - Get first line
;   $3B: LINE_CLEAR_ALL     - Clear entire program (NEW)
;   $3C: LINE_COUNT         - Count total lines
;   $3F: PROGRAM_END        - ELM points to end of program
;
; === GOSUB/RETURN STACK (AH=$40-$4F) ===
;   AH=$40: GOSUB_PUSH     - Push return address for GOSUB
;   AH=$41: GOSUB_POP      - Pop return address for RETURN
;   AH=$42: GOSUB_CLEAR    - Clear GOSUB stack
;
; === FOR/NEXT STACK (AH=$50-$5F) ===
;   AH=$50: FOR_PUSH       - Push FOR loop context
;   AH=$51: FOR_POP        - Pop FOR loop context
;   AH=$52: FOR_PEEK       - Peek at current FOR loop
;   AH=$53: FOR_CLEAR      - Clear FOR stack
;   AH=$54: FOR_FIND_VAR   - Find FOR loop by variable letter
;
; === INPUT/OUTPUT HELPERS (AH=$60-$6F) ===
;   AH=$60: IO_GETCHAR     - Get next character from input buffer
;   AH=$61: IO_PUTCHAR     - Output character to screen
;   AH=$62: IO_GETNUM      - Parse number from input
;   AH=$63: IO_PUTNUM      - Output number to screen
;   AH=$64: IO_NEWLINE     - Output CR/LF
;   AH=$65: IO_GETLINE     - Get line of input from use/publir
;   AH=$66: IO_PRINT_STR   - Print string at pointer
;   AH=$67: IO_PROCESS_STRING - Process a string in quotes for printing.
;   AH=$68: IO_INPUT - for the INPUT command. Simple string input.
;
; === STRING/TOKEN HELPERS (AH=$70-$8F) ===
;   AH=$70: STR_COMPARE    - Compare strings for keyword matching
;   AH=$71: STR_TO_UPPER   - Convert string to uppercase
;   AH=$72: STR_SKIP_SPACE - Skip whitespace in ELM string
;   AH=$73: STR_IS_DIGIT   - Check if character is digit
;   AH=$74: STR_IS_ALPHA   - Check if character is letter
;   AH=$75: STR_LENGTH     - Get string length
;   AH=$76: STR_COPY       - Copy string from src to dst
;   AH=$77: STR_CONCAT     - Concatenate two strings
;   AH=$78: STR_LEFT       - Extract left N characters (LEFT$)
;   AH=$79: STR_RIGHT      - Extract right N characters (RIGHT$)
;   AH=$7A: STR_MID        - Extract middle substring (MID$)
;   AH=$7B: STR_CHR        - Convert byte to character (CHR$)
;   AH=$7C: STR_ASC        - Convert character to byte (ASC)
;   AH=$7D: STR_VAL        - Convert string to number (VAL)
;   AH=$7E: STR_STR        - Convert number to string (STR$)
;   AH=$7F: STR_FIND       - Find substring within string (INSTR)
;   AH=$80: STR_LTRIM      - Trim leading whitespace
;   AH=$81: STR_RTRIM      - Trim trailing whitespace
;   AH=$82: STR_TRIM       - Trim both leading and trailing whitespace
;   AH=$83: STR_REVERSE    - Reverse a string
;   AH=$84: STR_REPEAT     - Repeat character N times (STRING$)
;   AH=$85: STR_SPLIT      - Split string at delimiter
;   AH=$86: STR_REPLACE    - Replace substring with another
;   AH=$87-$8F: (Reserved for future string operations)
;
; === ARITHMETIC/LOGIC HELPERS (AH=$90-$9F) ===
;   AH=$90: MATH_ADD       - Pop two values, push sum
;   AH=$91: MATH_SUB       - Pop two values, push difference
;   AH=$92: MATH_MUL       - Pop two values, push product
;   AH=$93: MATH_DIV       - Pop two values, push quotient
;   AH=$94: MATH_NEG       - Negate top of stack
;   AH=$95: MATH_RAND      - Generate random integer
;   AH=$96: MATH_RND       - Generate random number between 0 and 1
;
; === ADVANCED MATH (AH=$96-$9F) ===
;   AH=$9A: MATH_MOD        - Modulo (remainder)
;   AH=$9B: MATH_ABS        - Absolute value
;   AH=$9C: MATH_MIN        - Minimum of two values
;   AH=$9D: MATH_MAX        - Maximum of two values
;   AH=$9E: MATH_SGN        - Sign (-1, 0, or 1)
;
; === COMPARISON HELPERS (AH=$A0-$AF) ===
;   AH=$A0: CMP_EQ         - Compare equal
;   AH=$A1: CMP_NE         - Compare not equal
;   AH=$A2: CMP_LT         - Compare less than
;   AH=$A3: CMP_GT         - Compare greater than
;   AH=$A4: CMP_LE         - Compare less than or equal
;   AH=$A5: CMP_GE         - Compare greater than or equal
;   AH=$A6-$AF: (Reserved)
;
; === ERROR HANDLING (AH=$B0-$BF) ===
;   AH=$B0: ERROR_SET      - Set error flag and message
;   AH=$B1: ERROR_GET      - Get current error code
;   AH=$B2: ERROR_CLEAR    - Clear error flag
;   AH=$B3: ERROR_PRINT    - Print error message
;   AH=$B4-$BF: (Reserved)
;;
;
; === MEMORY OPERATIONS (AH=$C0-$CF) ===
;   AH=$C0: MEM_LOAD_BYTE   - Load byte from [addr on stack]
;   AH=$C1: MEM_STORE_BYTE  - Store byte to [addr on stack]
;   AH=$C2: MEM_LOAD_WORD   - Load word from [addr on stack]
;   AH=$C3: MEM_STORE_WORD  - Store word to [addr on stack]
;   AH=$C4: ADDR_ADD        - Pointer arithmetic (base + offset)
;   AH=$C5: ADDR_SCALE      - Scale index (index * size)
;
; === LOCAL VARIABLE FRAMES (AH=$D0-$DF) ===
;   AH=$D0: FRAME_ALLOC     - Allocate stack frame
;   AH=$D1: FRAME_FREE      - Free stack frame
;   AH=$D2: LOCAL_GET       - Get local variable by offset
;   AH=$D3: LOCAL_SET       - Set local variable by offset
;   AH=$D4: PARAM_GET       - Get function parameter
;   AH=$D5: PARAM_SET       - Set function parameter
;
; === LOGICAL/BITWISE OPERATIONS (AH=$E0-$EF) ===
;   AH=$E0: LOG_AND         - Logical AND (0/1 values)
;   AH=$E1: LOG_OR          - Logical OR (0/1 values)
;   AH=$E2: LOG_NOT         - Logical NOT (0/1 values)
;   AH=$E3: BIT_AND         - Bitwise AND
;   AH=$E4: BIT_OR          - Bitwise OR
;   AH=$E5: BIT_XOR         - Bitwise XOR
;   AH=$E6: BIT_NOT         - Bitwise NOT (complement)
;   AH=$E7: BIT_SHL         - Bitwise shift left
;   AH=$E8: BIT_SHR         - Bitwise shift right
;
; === TYPE CONVERSION (AH=$EA-$EF) ===
;   AH=$EA: CONV_BYTE_WORD  - Convert byte to word (zero-extend)
;   AH=$EB: CONV_WORD_BYTE  - Convert word to byte (truncate)
;   AH=$EC: CONV_SIGNED     - Sign-extend byte to word


; === IL INTERPRETER CORE (AH=$F0-$FF) ===
;   AH=$F0: IL_INIT        - Initialize IL interpreter
;   AH=$F1: IL_LOAD        - Load IL code into memory
;   AH=$F2: IL_FETCH       - Fetch next IL instruction
;   AH=$F3: IL_DISPATCH    - Dispatch IL instruction
; ============================================================================

; ============================================================================
; INT 05h - PALO ALTO TINY BASIC SYSTEM
; Memory map and constants
; ============================================================================
; *** Note: If you make changes here please reflect them in memorymap.sda

.equ PATB_TBUF          $01DB00   ; tokenizer buffer (as listed in memory map) CHECKME: not really being used?
.equ PATB_VARIABLES     $01EE00   ; 26 vars × 2 bytes = 52 bytes ($01EE00-$01EE33)
.equ PATB_EXPR_STACK    $01EE34   ; Expression stack, 64 bytes ($01EE34-$01EE73)
.equ PATB_EXPR_SP       $01EE74   ; Expression stack pointer (2 bytes)
.equ PATB_GOSUB_STACK   $01EE76   ; GOSUB stack, 48 bytes (16 levels × 3 bytes)
.equ PATB_GOSUB_SP      $01EEA6   ; GOSUB stack pointer (2 bytes)
.equ PATB_FOR_STACK     $01EEA8   ; FOR stack, 40 bytes (8 levels × 5 bytes)
.equ PATB_FOR_SP        $01EED0   ; FOR stack pointer (2 bytes)
.equ PATB_GOTO_TARGET   $01EED2   ; 2 bytes: 0 = no goto, nonzero = target line#
.equ PATB_STATUS        $01EED4   ; 1 byte
                                  ; Bit 0: Running (1) / Stopped (0)
                                  ; Bit 1: Error flag
                                  ; Bit 2: Break flag
.equ PATB_RANDSEED      $01EED5   ; 2 bytes
.equ PATB_PROGRAM_PTR   $01EED7   ; 3 bytes
.equ PATB_IL_PTR        $01EEDA   ; 3 bytes
.equ PATB_INPUT_PTR     $01EEDD   ; 3 bytes
.equ PATB_BANK          $01EEE0   ; 1 byte
.equ PATB_DIM_NAMEPTR   $01EEE1   ; 3 bytes - saved var name pointer for DIM loop
.equ PATB_VARTOP        $01EEE4   ; 3 bytes - bottom of variable storage (grows down)
.equ PATB_DATA_PTR      $01EEE7   ; 3 bytes - for DATA and READ statements
.equ PATB_MS_PTR        $01EEEA   ; 3 bytes - multiline statement pointer

; RESERVED: $01EEED-$01EEFF = 19 bytes free

; Refer to memorymap.sda if these change
.equ PATB_PROGRAM_BASE  $000100   ; Start of program storage (Bank 3, offset $100)
.equ PATB_PROGRAM_END   $00FFFF   ; End (almost 64KB for programs)

; ============================================================================
; INT 10h - KEYBOARD / TERMINAL SERVICES
; PC BIOS-style keyboard interrupt handler
; ============================================================================
; Function dispatch via AH register:
;   AH=00h: Read character (non-blocking)
;   AH=01h: Check if key available (in keyboar buffer)
;   AH=02h: Read character (blocking)
;   AH=03h: Get keyboard flags/shift state
;   AH=04h: Clear keyboard buffer
;   AH=05h: Push character into buffer

;   AH=10h: Clear screen (blank characters only)
;   AH=11h: Write character at cursor location
;   AH=12h: Read character at cursor location
;   AH=13h: Set color data at cursor location
;   Ah=14h: Get color data at cursor location
;   AH=15h: Set cursor position
;   AH=16h: Get cursor position
;   AH=17h: Print character at cursor (teletype)
;   AH=18h: Print string at cursor
;   Ah=19h: Read string between X,Y and cursor
;   AH=1Ah: Carriage Return
;   AH=1Bh: Issue Linefeed
;   AH=1Ch: Scroll screen up (linefeed_push forced linefeed)

;   AH=20h: Cursor Left (move cursor left one column)
;   AH=21h: Cursor Right (move cursor right one column)
;   AH=22h: Cursor Up (move cursor up one row)
;   AH=23h: Cursor Down (move cursor down one row)
;   AH=24h: Bell (audible beep - optional)
;   AH=25h: Backspace (move left and erase character)
;   AH=26h: Delete (pull line left from cursor to end of line)
;   AH=27h: Tab (move to next tab stop)
;   AH=28h: Home (move cursor to 0,0)
;   AH=29h: Advance Cursor with Line Link Hook (links to term.sda)
;   links to term.sda:
;   AH=$2A: int10_cursor_blink -- Blink cursor (main loop)
;   AH=$2B: int10_cursor_on -- Turn cursor on
;   AH=$2C: int10_cursor_off -- Turn cursor off
;   AH=$2D: int10_cursor_toggle -- Toggle cursor
;   AH=$2E: int10_set_cursor_colors -- Set char/cursor colors

;   AL=30h: Clear to End of Line (erase from cursor to end of line)
;   AL=31h: Clear to End of Screen (erase from cursor to end of screen)
;   AL=32h: Insert Line (insert blank line at cursor row)
;   AL=33h: Delete Line (delete line at cursor row)
;   AL=34h: link line
;   AL=35h:
;   AL=36h:
;   AL=37h:
;   AH=38h: Scroll Line Links

;   AH=40h: Set video mode
;   AH=41h: Get current video mode
;   AH=42h: Get MAX ROWS (mode 0:23, 1:25, 2:25, 3: 30)
;   AH=43h: Get MAX COLS (mode 0:22, 1:40, 2:80, 3: 120)
;   AH=44h: Get pointer to VIDEO_TEXT_BASE for current mode (or start of framebuffer for graphics modes)
;   AH=45h: Get pointer to VIDEO_COLOR_BASE for current mode (or palette space for graphics modes)
;   AH=46h: is text mode?
;   AH=47h: get char rom base

Please see the interrupt table and function discussion in Appendix 5 Sound System of the SD-8516 Programmer's Reference Guide.

• INT 12h String Services

; ============================================================================
; INT 13h - MATH SERVICES
; Stellar BIOS Math operations interrupt handler
; ============================================================================
; Function dispatch via AH register:
;   AH=00h: Random number (16-bit)
;   AH=01h: Random number in range
;   AH=02h: Seed random number generator
;   AH=03h: Square root (integer)
;   AH=04h: Power (base^exponent)
;   AH=05h: Sine (8-bit fixed point)
;   AH=06h: Cosine (8-bit fixed point)
;   AH=07h: Tangent (8-bit fixed point)
;   AH=08h: Arctangent2 (for angles)
;   AH=09h: Absolute value
;   AH=0Ah: Min of two values
;   AH=0Bh: Max of two values
;   AH=0Ch: Clamp value to range
;
; ============================================================================

; ============================================================================
; INT 18h - GRAPHICS SERVICES
; Stellar BIOS Graphics operations interrupt handler
; ============================================================================
; Function dispatch via AH register:
;   AH=00h: reserved (for set graphics mode see int 10h ah=40h)
;   AH=01h: Plot MODE 3 pixel (4bpp)
;   AH=02h: Plot MODE 4 pixel (8bpp)
;   AH=11h: Get MODE 3 pixel (4bpp)
;   AH=12h: Get MODE 4 pixel (8bpp)

;   AH=20h: Clear screen (fill with color)
;   AH=21h: Draw line
;   AH=22h: Draw rectangle (outline)
;   AH=23h: Fill rectangle
;   AH=24h: Draw circle (outline)
;   AH=25h: Fill circle
;   AH=26h: Draw ellipse
;   AH=30h: Blit sprite/image

;   AH=40h: Set color mode for video mode
;   AH=41h: Get current color mode
;   AH=42h: Set palette entry (AL=0, use 0-15. Anything else, use 0-255)
;   AH=43h: Get palette entry (AL=0, use 0-15. Anything else, use 0-255)
;   AH=44h: Load palette (AL=0, use 16color, anything else, all 256. ELM=source for palette data)
;   AH=45h: Set draw color (deprecated for now)
;   AH=46h: Flood fill (PAINT) (deprecated for now)

;   AH=50h: Scroll region
;   AH=51h: Copy region
; ============================================================================