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. However, the INT 05h system was done first so it represents a general convention you can expect going forward.

• 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, ELD (see calling convention)

LDAL #'A'
LDB #1234
LDAH $11        ; VAR_SET
INT $05

LDELD @string
LDAH $62        ; IO_GETNUM  
INT $05
; B has parsed number

First let's discuss the memory map used by our implementation of Pao Alto Tiny Basic.

Found at $01ED00-$01EDFF, 256 bytes available.

• .equ PATB_VARIABLES $01ED00 ; 26 vars × 2 bytes = 52 bytes ($01ED00-$01ED33)
• .equ PATB_EXPR_STACK $01ED40 ; Expression stack, 64 bytes ($01ED40-$01ED7F)
• .equ PATB_EXPR_SP $01ED80 ; Expression stack pointer (2 bytes)
• .equ PATB_GOSUB_STACK $01ED82 ; GOSUB stack, 32 bytes (16 levels × 2 bytes)
• .equ PATB_GOSUB_SP $01EDA2 ; GOSUB stack pointer (2 bytes)
• .equ PATB_FOR_STACK $01EDA4 ; FOR stack, 40 bytes (8 levels × 5 bytes)
• .equ PATB_FOR_SP $01EDCC ; FOR stack pointer (2 bytes)
• .equ PATB_PROGRAM_PTR $01EDCE ; Current program line pointer (3 bytes)
• .equ PATB_IL_PTR $01EDD1 ; Current IL instruction pointer (3 bytes)
• .equ PATB_INPUT_PTR $01EDD4 ; Current input pointer (3 bytes)
• .equ PATB_RANDSEED $01EDD7 ; Random seed (2 bytes)
• .equ PATB_STATUS $01EDD9 ; Status flags (1 byte)
• ; Bit 0: Running (1) / Stopped (0)
• ; Bit 1: Error flag
• ; Bit 2: Break flag

INT 05h - PALO ALTO TINY BASIC SYSTEM Complete implementation for PATB IL interpreter

; 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 ; ;

; 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 ; ;

; 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 ; ;

; AH=$30: LINE_FIND - Find line by number ; AH=$31: LINE_NEXT - Get next line ; AH=$32: LINE_INSERT - Insert or replace line ; AH=$33: LINE_DELETE - Delete line by number ; AH=$34: LINE_FIRST - Get first line in program ; AH=$35: LINE_CLEAR_ALL - Clear entire program (NEW) ; AH=$36: LINE_COUNT - Count total lines ; ;

; 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 ; ;

; 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 ; ;

; 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 user ; AH=$66: IO_PRINT_STR - Print string at pointer ; ;

; 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 string ; AH=$73: STR_IS_DIGIT - Check if character is digit ; AH=$74: STR_IS_ALPHA - Check if character is letter ; ;

; AH=$80: MATH_ADD - Pop two values, push sum ; AH=$81: MATH_SUB - Pop two values, push difference ; AH=$82: MATH_MUL - Pop two values, push product ; AH=$83: MATH_DIV - Pop two values, push quotient ; AH=$84: MATH_NEG - Negate top of stack ; AH=$85: MATH_RND - Generate random number ; ;

; AH=$90: CMP_EQ - Compare equal (pop 2, push 1 if true, 0 if false) ; AH=$91: CMP_NE - Compare not equal ; AH=$92: CMP_LT - Compare less than ; AH=$93: CMP_GT - Compare greater than ; AH=$94: CMP_LE - Compare less than or equal ; AH=$95: CMP_GE - Compare greater than or equal ; ;

; AH=$A0: ERROR_SET - Set error flag and message ; AH=$A1: ERROR_GET - Get current error code ; AH=$A2: ERROR_CLEAR - Clear error flag ; AH=$A3: ERROR_PRINT - Print error message ; ;

; 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 10h - VIDEO SERVICES
  ; Stellar BIOS Video interrupt handler
  ; ============================================================================
  ; Function dispatch via AH register:
  ;   AH=00h: Set video mode
  ;   AH=01h: Set cursor shape (reserved)
  ;   AH=02h: Set cursor position
  ;   AH=03h: Get cursor position
  ;   AH=0Eh: Write character at cursor (teletype)
  ;   AH=0Fh: Get current video mode
  ;   AH=13h: Write string at cursor
  ;   AH=14h: Scroll screen up
  ;   AH=15h: Clear screen
  ;   AH=16h: Get video parameters (rows, cols)
  ; ============================================================================

• Returns: AL is 00 on success, 01 if the mode failed to init.

Available modes:

• Mode 0: 22×23 console
• Mode 1: 40×25 console
• Mode 2: 80×25 console
• Mode 3: still thinking, graphics, or superterminal? 1280×800 “128×40“font via 10×20 charset??

• returns character code in AL and keyboard flags in BL.

• returns character code in AL, keyboard flags in BL, and time spent waiting in ms in C

• returns keyboard flags in AL.

• returns the number of keys waiting in AL. I.E. 0=no keys available, 1+= has keys waiting.