apple2a/runtime.c

#include <string.h>
#include "runtime.h"

// Max number of nested FOR loops. This value matches AppleSoft BASIC.
#define MAX_FOR 10

#define CURSOR_GLYPH 127
#define SCREEN_HEIGHT 24
#define SCREEN_WIDTH 40
#define SCREEN_STRIDE (3*SCREEN_WIDTH + 8)
#define MIXED_TEXT_HEIGHT 4
#define MIXED_GRAPHICS_HEIGHT (SCREEN_HEIGHT - MIXED_TEXT_HEIGHT)
#define CLEAR_CHAR (' ' | 0x80)

#define TEXT_OFF_SWITCH ((uint8_t *) 49232U)
#define TEXT_ON_SWITCH ((uint8_t *) 49233U)
#define MIXED_OFF_SWITCH ((uint8_t *) 49234U)
#define MIXED_ON_SWITCH ((uint8_t *) 49235U)
#define HIRES_OFF_SWITCH ((uint8_t *) 49238U)
#define HIRES_ON_SWITCH ((uint8_t *) 49239U)

/**
 * Run-time stack of FOR loops.
 */
typedef struct {
    // Address (in the zero page) of the loop variable.
    uint8_t var_address;

    // End value.
    int16_t end_value;

    // Step.
    int16_t step;

    // Address of the top of the loop to jump back to.
    uint16_t loop_top_addr;
} ForInfo;

// Location of cursor in logical screen space.
uint16_t g_cursor_x = 0;
uint16_t g_cursor_y = 0;
// Whether the cursor is being displayed.
uint16_t g_showing_cursor = 0;
// Character at the cursor location.
uint8_t g_cursor_ch = 0;

// Whether in low-res graphics mode.
uint8_t g_gr_mode = 0;

// 4-bit low-res color.
uint8_t g_gr_color_high = 0; // High nybble.
uint8_t g_gr_color_low = 0;  // Low nybble.

// List of variable names, two bytes each, in the same order they are
// in the zero page (starting at FIRST_VARIABLE). Two nuls means an unused
// slot. One-letter variable names have a nul for the second character.
uint8_t g_variable_names[MAX_VARIABLES*2];

// Stack of FOR loops.
ForInfo g_for_info[MAX_FOR];
uint8_t g_for_count;

/**
 * Clear the FOR stack.
 */
void clear_for_stack(void) {
    g_for_count = 0;
}

/**
 * Clear out the values of all variables and generally initialize runtime
 * state.
 */
void initialize_runtime(void) {
    memset((void *) FIRST_VARIABLE, 0, MAX_VARIABLES*2);
    clear_for_stack();
}

/**
 * Return the memory location of the zero-based (x,y) position on the screen.
 */
static uint8_t *screen_pos(uint16_t x, uint16_t y) {
    int16_t block = y >> 3;
    int16_t line = y & 0x07;

    return TEXT_PAGE1_BASE + line*SCREEN_STRIDE + block*SCREEN_WIDTH + x;
}

/**
 * Return the memory location of the cursor.
 */
uint8_t *cursor_pos(void) {
    return screen_pos(g_cursor_x, g_cursor_y);
}

/**
 * Shows the cursor. Safe to call if it's already showing.
 */
void show_cursor(void) {
    if (!g_showing_cursor) {
        uint8_t *pos = cursor_pos();
        g_cursor_ch = *pos;
        *pos = CURSOR_GLYPH | 0x80;
        g_showing_cursor = 1;
    }
}

/**
 * Hides the cursor. Safe to call if it's not already shown.
 */
void hide_cursor(void) {
    if (g_showing_cursor) {
        uint8_t *pos = cursor_pos();
        *pos = g_cursor_ch;
        g_showing_cursor = 0;
    }
}

/**
 * Moves the cursor to the specified location, where X
 * is 0 to 39 inclusive, Y is 0 to 23 inclusive.
 */
void move_cursor(int16_t x, int16_t y) {
    hide_cursor();
    g_cursor_x = x;
    g_cursor_y = y;
}

/**
 * Blanks out the rest of the line, from the cursor (inclusive) on.
 * Does not move the cursor.
 */
void clear_to_eol(void) {
    uint8_t *pos = cursor_pos();

    hide_cursor();
    memset(pos, CLEAR_CHAR, SCREEN_WIDTH - g_cursor_x);
}

/**
 * Clear the screen with non-reversed spaces.
 */
void home(void) {
    if (g_gr_mode) {
        int i;

        for (i = MIXED_GRAPHICS_HEIGHT; i < SCREEN_HEIGHT; i++) {
            memset(screen_pos(0, i), CLEAR_CHAR, SCREEN_WIDTH);
        }

        move_cursor(0, MIXED_GRAPHICS_HEIGHT);
    } else {
        memset(TEXT_PAGE1_BASE, CLEAR_CHAR, SCREEN_STRIDE*8);
        move_cursor(0, 0);
    }
}

/**
 * Screen the screen up one line, blanking out the bottom
 * row. Does not affect the cursor.
 */
static void scroll_up(void) {
    int i;
    int first_line = g_gr_mode ? MIXED_GRAPHICS_HEIGHT : 0;
    uint8_t *previous_line = 0;

    for (i = first_line; i < SCREEN_HEIGHT; i++) {
        uint8_t *this_line = screen_pos(0, i);
        if (i > first_line) {
            memmove(previous_line, this_line, SCREEN_WIDTH);
        }
        previous_line = this_line;
    }

    memset(previous_line, CLEAR_CHAR, SCREEN_WIDTH);
}

/**
 * Print a single newline.
 */
void print_newline(void) {
    if (g_cursor_y == SCREEN_HEIGHT - 1) {
        // Scroll.
        hide_cursor();
        scroll_up();
        move_cursor(0, g_cursor_y);
    } else {
        move_cursor(0, g_cursor_y + 1);
    }
}

/**
 * Prints the character and advances the cursor. Handles newlines.
 */
void print_char(uint8_t c) {
    uint8_t *loc = cursor_pos();

    if (c == '\n') {
        print_newline();
    } else {
        // Print character.
        *loc = c | 0x80;

        // Advance cursor or wrap.
        if (g_cursor_x == SCREEN_WIDTH - 1) {
            print_newline();
        } else {
            move_cursor(g_cursor_x + 1, g_cursor_y);
        }
    }
}

/**
 * Print a string at the cursor.
 */
void print(uint8_t *s) {
    while (*s != '\0') {
        print_char(*s++);
    }
}

/**
 * Print an unsigned integer.
 */
void print_uint(uint16_t i) {
    // Is this the best way to do this? I've seen it done backwards, where
    // digits are added to a buffer least significant digit first, then reversed,
    // but this seems faster.
    char printed = 0;

    if (i >= 10000) {
        int16_t r = i / 10000;
        print_char('0' + r);
        i -= r*10000;
        printed = 1;
    }
    if (i >= 1000 || printed) {
        int16_t r = i / 1000;
        print_char('0' + r);
        i -= r*1000;
        printed = 1;
    }
    if (i >= 100 || printed) {
        int16_t r = i / 100;
        print_char('0' + r);
        i -= r*100;
        printed = 1;
    }
    if (i >= 10 || printed) {
        int16_t r = i / 10;
        print_char('0' + r);
        i -= r*10;
    }
    print_char('0' + i);
}

/**
 * Print a signed integer.
 */
void print_int(int16_t i) {
    if ((i & 0x8000) != 0) {
        print_char('-');
        i = -i;
    }

    print_uint((uint16_t) i);
}

/**
 * Print an error message, optionally with a line number if it's
 * not INVALID_LINE_NUMBER.
 */
static void generic_error_message(uint8_t *message, uint16_t line_number) {
    print("\n?");
    print(message);

    if (line_number != INVALID_LINE_NUMBER) {
        print(" IN ");
        print_uint(line_number);
    }
}

/**
 * Display a syntax error message.
 */
void syntax_error(uint16_t line_number) {
    generic_error_message("SYNTAX ERROR", line_number);
}

/**
 * Display an error for a GOTO that went to a line that doesn't exist.
 */
void undefined_statement_error(uint16_t line_number) {
    generic_error_message("UNDEF'D STATEMENT ERROR", line_number);
}

/**
 * Display an out-of-memory error, which could also mean that various
 * stacks have been overflowed.
 */
void out_of_memory_error(uint16_t line_number) {
    generic_error_message("OUT OF MEMORY ERROR", line_number);
}

/**
 * Display an error for when the user does a NEXT without a matching FOR.
 */
void next_without_for_error(uint16_t line_number) {
    generic_error_message("NEXT WITHOUT FOR ERROR", line_number);
}

/**
 * Switch to graphics mode.
 */
void gr_statement(void) {
    if (!g_gr_mode) {
        int i;
        // Mixed text and lo-res graphics mode.

        hide_cursor();

        *TEXT_OFF_SWITCH = 0;
        *MIXED_ON_SWITCH = 0;

        // Clear the graphics area.
        for (i = 0; i < MIXED_GRAPHICS_HEIGHT; i++) {
            memset(screen_pos(0, i), 0, SCREEN_WIDTH);
        }

        // Move the cursor to the text window.
        if (g_cursor_y < MIXED_GRAPHICS_HEIGHT) {
            move_cursor(0, MIXED_GRAPHICS_HEIGHT);
        }

        g_gr_mode = 1;
    }
}

/**
 * Switch to text mode.
 */
void text_statement(void) {
    if (g_gr_mode) {
        // Text mode.
        *TEXT_ON_SWITCH = 0;

        hide_cursor();

        g_gr_mode = 0;
    }
}

/**
 * Set the low-res color.
 */
void color_statement(uint16_t color) {
    g_gr_color_high = (uint8_t) ((color << 4) & 0xF0);
    g_gr_color_low = (uint8_t) (color & 0x0F);
}

/**
 * Plot a pixel in low-res graphics mode.
 */
void plot_statement(uint16_t x, uint16_t y) {
    uint8_t *pos = screen_pos(x, y >> 1);

    if ((y & 1) == 0) {
        // Even, bottom pixel.
        *pos = (*pos & 0xF0) | g_gr_color_low;
    } else {
        // Odd, top pixel.
        *pos = (*pos & 0x0F) | g_gr_color_high;
    }
}

/**
 * Find a FOR loop info structure by variable address, or null if not found.
 */
static ForInfo *find_for_info(uint16_t var_address) {
    int i;
    ForInfo *f;

    // Work backwards through the stack, since the variable we're looking
    // for is likely to be the most recent one.
    f = &g_for_info[g_for_count - 1];
    for (i = g_for_count - 1; i >= 0; i--) {
        if (f->var_address == var_address) {
            // Found it.
            return f;
        }

        f -= 1;
    }

    return 0;
}

/**
 * Remove any FOR loop for this variable anywhere in the stack, if any.
 */
static void remove_for_info(uint16_t var_address) {
    ForInfo *f = find_for_info(var_address);

    if (f != 0) {
        // Compute the index of this entry.
        int index = f - g_for_info;

        // Shift the rest over.
        memmove(f, f + 1, sizeof(ForInfo)*(g_for_count - index - 1));
        g_for_count -= 1;
    }
}

/**
 * Push a FOR statement on the stack.
 */
void for_statement(uint16_t line_number, uint16_t var_address, int16_t end_value, int16_t step,
        uint16_t loop_top_addr) {

    // First, kill any existing loop for this variable.
    remove_for_info(var_address);

    // Add the loop to our stack.
    if (g_for_count == MAX_FOR) {
        // TODO should quit program. Return a failure value, and have called return.
        out_of_memory_error(line_number);
    } else {
        ForInfo *f = &g_for_info[g_for_count++];

        f->var_address = var_address;
        f->end_value = end_value;
        f->step = step;
        f->loop_top_addr = loop_top_addr;
    }
}

/**
 * Handle a NEXT statement. Returns the address to jump to at the top of the loop,
 * or zero to not jump.
 */
uint16_t next_statement(uint16_t line_number, uint16_t var_address) {
    ForInfo *f;
    uint16_t jump_addr = 0;

    if (var_address == 0) {
        // Pick top of stack.
        if (g_for_count == 0) {
            // Stack is empty.
            f = 0;
        } else {
            f = &g_for_info[g_for_count - 1];
        }
    } else {
        f = find_for_info(var_address);
    }

    if (f == 0) {
        next_without_for_error(line_number);
    } else {
        uint16_t *var;

        // Pop off every loop below us in the stack.
        g_for_count = f - g_for_info + 1;

        // Step the loop variable.
        var = (uint16_t *) f->var_address;
        *var += f->step;
        // TODO if the step is negative, switch inequality here:
        if (*var > f->end_value) {
            // We're done, remove our FOR loop.
            g_for_count -= 1;

            // Continue after the NEXT statement.
        } else {
            // Loop back to the top of the NEXT statement.
            jump_addr = f->loop_top_addr;
        }
    }

    return jump_addr;
}