[screensavers] / hacks / NEDsim / NEDsim.c

/* (c) 2021 Aaron Taylor <ataylor at subgeniuskitty dot com>                  */
/* See LICENSE.txt file for copyright and license details.                    */

#include "screenhack.h"
#include "simulator.h"
#include "ned_programs/embedded_ned_program_declarations.h"

/* -------------------------------------------------------------------------- */
/* Front Panel Layout                                                         */
/* -------------------------------------------------------------------------- */

// All of these values are in units of 'cells', not 'pixels'. Where applicable,
// coordinates refer to the upper-left corner of a cell. See the files
// `layout_reference.*` for details.

#define OVERALL_WIDTH_IN_CELLS      70
#define HEADER_HEIGHT_IN_CELLS      14
#define FOOTER_HEIGHT_IN_CELLS      2

#define LOGO_X_OFFSET               2
#define LOGO_Y_OFFSET               2
#define LOGO_WIDTH                  20
#define LOGO_NAME_HEIGHT            6
#define LOGO_WEBSITE_HEIGHT         2

#define HALT_X_OFFSET               26
#define HALT_Y_OFFSET               2
#define HALT_WIDTH                  6
#define HALT_LIGHT_HEIGHT           6
#define HALT_LABEL_HEIGHT           2

#define WORDLINE_BITS_PER_STRIPE    4
#define WORDLINE_WIDTH              32
#define WORDLINE_HEIGHT             1

#define PC_X_OFFSET                 36
#define PC_Y_OFFSET                 7
#define PC_WIDTH                    32
#define PC_LABEL_HEIGHT             2
#define PC_LIGHT_HEIGHT             1

#define SC_X_OFFSET                 42
#define SC_Y_OFFSET                 2
#define SC_WIDTH                    5
#define SC_LABEL_HEIGHT             2
#define SC_LIGHT_HEIGHT             1

#define PSW_N_X_OFFSET              51
#define PSW_Z_X_OFFSET              58
#define PSW_Y_OFFSET                2
#define PSW_LABEL_WIDTH             3
#define PSW_LABEL_HEIGHT            2
#define PSW_LIGHT_WIDTH             1
#define PSW_LIGHT_HEIGHT            1

#define STACK_X_OFFSET              2
#define STACK_Y_OFFSET              12
#define STACK_WIDTH                 32
#define STACK_LABEL_HEIGHT          2
#define STACK_LIGHT_HEIGHT          1

#define HEAP_X_OFFSET               36
#define HEAP_Y_OFFSET               12
#define HEAP_WIDTH                  32
#define HEAP_LABEL_HEIGHT           2
#define HEAP_LIGHT_HEIGHT           1

/* -------------------------------------------------------------------------- */
/* Data Structures                                                            */
/* -------------------------------------------------------------------------- */

struct NEDsim {
    // Various X resources
    Display * dpy;
    Window    win;
    GC        gc;

    // Tracks the width and height (in pixels) of 'win' on 'dpy'.
    int dpy_width, dpy_height;

    // To ensure the NED display is always synchronized/valid, we
    // double-buffer. This is that buffer.
    Pixmap panel;

    // The front panel is defined in a fixed grid of 'cells' which are then
    // scaled and projected onto the actual screen. This variable tracks the
    // size of one 'cell' in screen pixels.
    int cell_size;
    // Like above, this variable tracks the scaled size (in pixels) of cell
    // borders when projecting them on the screen.
    int border_size;

    // Since the panel will be smaller than the display window, these two
    // variable track the location (in pixels) of the upper left corner of the
    // NED panel (pretending it didn't have rounded corners) relative to the
    // display window's origin.
    int origin_x_offset, origin_y_offset;

    // Delay (in microseconds) between clock cycles in the NED CPU.
    size_t delay;

    // Since the heap and stack areas are resized to fit the display, this
    // variable tracks their height.
    int num_data_rows;

    // This is an index into the color_list[] array, selecting a color scheme.
    size_t color_index;

    // If the display is unsuitable (e.g. too small), this boolean is 'true'.
    // With this, we blank the display but keep the simulation itself running,
    // allowing the ongoing simulation to display when the situation is
    // corrected, and allowing us to avoid boring the user with unnecessary
    // simulation restarts.
    Bool suitable_display;

    // The font size is dynamically adjusted based on display size. This
    // variable stores the full font description, including the correct font
    // size, all in the X11 font specifier format. For example,
    //             -*-helvetica-*-r-*-*-72-*-*-*-*-*-*-*
    // if the display were using a 72 point helvetica font without emphasis.
    char * current_font;

    // Contains the address of the top of the heap window. In total, the heap
    // window will display from `heap_window_offset` to
    // `heap_window_offset+num_data_rows`.
    uint32_t heap_window_offset;

    // This struct contains all machine state for the simulated NED computer.
    struct NEDstate * nedstate;
};

struct color_rgb {
    // The type 'unsigned short' comes from the XColor struct definition.
    // The red, green, and blue values are always in the range 0 to 65535
    // inclusive, independent of the number of bits actually used in the
    // display hardware. The server scales these values to the range used
    // by the hardware. Black is represented by (0,0,0), and white is
    // represented by (65535,65535,65535).
    unsigned short red, green, blue;
};

struct color_scheme {
    // The following entries define a full color scheme for a NED panel.
    struct color_rgb
        panel_bg,  // Empty screen space not taken up by the panel
        panel_fg,  // Panel body/faceplate
        light_on,  // Illuminated data indicator
        light_off, // Non-illuminated data indicator
        error_on,  // Illuminated error indicator
        error_off, // Non-illuminated error indicator
        primary,   // Primary panel color (see: logo background)
        secondary, // Secondary panel color (see: "HALT" text background)
        tertiary,  // Tertiary panel color (see: "subgeniuskitty.com" text background)
        border,    // Cell borders
        text;      // Panel text
};

// All included color schemes for NEDsim are defined in this array. To select a
// particular color scheme at runtime, use the `-color N` CLI option.
static struct color_scheme color_list[] = {
    { // This color scheme is inspired by the KI10 version of the PDP-10.
      // Many RGB values came from: http://www.chdickman.com/pdp8/DECcolors/
        {63479,63479,63479}, // 092-XXXX-123
        { 5140, 2056, 5654}, // 092-XXXX-152
        {65535,11051,15677}, // homemade, derived from 092-XXXX-139
        {20000,13107,12850}, // homemade, derived from 092-XXXX-154
        {65535,13107,12850}, // homemade, derived from 092-XXXX-154
        {20000,13107,12850}, // homemade, derived from 092-XXXX-154
        { 4112,12850,20046}, // 092-XXXX-145
        {12336,29555,37008}, // 092-XXXX-151
        {30326,24158, 6425}, // 092-XXXX-157
        {63479,63479,63479}, // 092-XXXX-123
        {63479,63479,63479}  // 092-XXXX-123
    },
    { // This color scheme is (very loosely) inspired by the first version of the PDP-11/70.
      // Many RGB values came from: http://www.chdickman.com/pdp8/DECcolors/
        {63479,63479,63479}, // 092-XXXX-123
        { 5140, 2056, 5654}, // 092-XXXX-152
        {51400,34952,26682}, // homemade, derived from 092-XXXX-136
        {20000,13107,12850}, // homemade, derived from 092-XXXX-154
        {65535,13107,12850}, // homemade, derived from 092-XXXX-154
        {20000,13107,12850}, // homemade, derived from 092-XXXX-154
        {28270, 8995,19532}, // 092-XXXX-140
        {40092,11051,15677}, // 092-XXXX-139
        {15000,21000, 4000}, // homemade, derived from 092-XXXX-129
        {63479,63479,63479}, // 092-XXXX-123
        {63479,63479,63479}  // 092-XXXX-123
    }
};

// Each instance of this struct will describe a single NED1 program embedded in
// NEDsim as a binary blob. See `ned_programs/README.md` for details.
struct ned_program {
    const uint8_t * binary_blob; // Pointer to raw NED1 machine code
    const size_t * blob_size;    // Size of raw NED1 machine code
    uint32_t heap_window_offset; // Location of window into NED system RAM
};

// All NED1 programs embedded in NEDsim should be manually added to this array.
// See `ned_programs/README.md` for details.
static struct ned_program ned_programs[] = {
    {
        integer_stack,
        &integer_stack_size,
        0x20000000
    },
    {
        long_division,
        &long_division_size,
        0x20000000
    }
};

/* -------------------------------------------------------------------------- */
/* Helper Functions                                                           */
/* -------------------------------------------------------------------------- */

// Fill in boilerplate when selecting colors.
#define COLOR(X) &color_list[nedsim->color_index].X

// Set foreground color for the current graphics context.
static void
set_color(struct NEDsim * nedsim, struct color_rgb * color)
{
    XColor temp;
    XWindowAttributes xgwa;

    XGetWindowAttributes(nedsim->dpy, nedsim->win, &xgwa);

    temp.red = color->red;
    temp.green = color->green;
    temp.blue = color->blue;

    XAllocColor(nedsim->dpy, xgwa.colormap, &temp);
    XSetForeground(nedsim->dpy, nedsim->gc, temp.pixel);
}

// Set font size to fill 'size' cells vertically, minus space for border and
// padding.  There is probably a better way to do this. The current method is
// horribly slow. Call this function as few times as possible.
static void
set_font_size(struct NEDsim * nedsim, int size)
{
    //                                                          vvv--- for border ---vvv    vvv--- for padding ---vvv
    int desired_height_in_pixels = (size * nedsim->cell_size) - (2 * nedsim->border_size) - (8 * nedsim->border_size);

    const char * font_size_prefix = "-*-helvetica-*-r-*-*-";
    const char * font_size_suffix = "-*-*-*-*-*-*-*";

    // In the buffer, we reserve 100 DIGITS for the font size integer.
    size_t buffer_size = strlen(font_size_prefix) + strlen(font_size_suffix) + 100;
    char * font_full_name = malloc(buffer_size);
    // Start with a 2pt font and work our way up.
    int font_size_in_points = 2;

    while (1) {
        // Load the font.
        snprintf(font_full_name, buffer_size, "%s%d%s", font_size_prefix, font_size_in_points, font_size_suffix);
        XFontStruct * font = XLoadQueryFont(nedsim->dpy, font_full_name);
        if (!font) {
            printf("WARNING: Unable to load font %s. Probably gonna look wonky.\n", font_full_name);
            font = XLoadQueryFont(nedsim->dpy, "fixed");
            break;
        }
        XSetFont(nedsim->dpy, nedsim->gc, font->fid);

        // Get the height.
        int direction, ascent, descent;
        XCharStruct overall;
        XTextExtents(font, "X", 1, &direction, &ascent, &descent, &overall);

        // Compare the height.
        int height = overall.ascent - overall.descent;
        if (height == desired_height_in_pixels) {
            break;
        } else if (height > desired_height_in_pixels) { 
            font_size_in_points--;
            snprintf(font_full_name, buffer_size, "%s%d%s", font_size_prefix, font_size_in_points, font_size_suffix);
            XFontStruct * font = XLoadQueryFont(nedsim->dpy, font_full_name);
            if (!font) {
                printf("WARNING: Unable to load font %s. Probably gonna look wonky.\n", font_full_name);
                font = XLoadQueryFont(nedsim->dpy, "fixed");
                break;
            }
            XSetFont(nedsim->dpy, nedsim->gc, font->fid);
            break;
        } else {
            font_size_in_points++;
        }
    }

    free(nedsim->current_font);
    nedsim->current_font = font_full_name;
}

// Unlike most functions in this program that input/output in units of 'cells',
// this function uses units of 'pixels' so the caller can track fractional cell
// usage (e.g. for centering the text).
static void
get_text_size(struct NEDsim * nedsim, const char * text, int * x_size, int * y_size)
{
    int direction, ascent, descent;
    XCharStruct overall;
    XFontStruct * font = XLoadQueryFont(nedsim->dpy, nedsim->current_font);
    XTextExtents(font, text, strlen(text), &direction, &ascent, &descent, &overall);
    *x_size = overall.width;
    *y_size = overall.ascent - overall.descent;
}

// When specifying the rectangular area to draw, all coordinates and sizes are
// in units of 'cells'.
static void
draw_rect_area(struct NEDsim * nedsim, struct color_rgb * color,
               size_t x_origin, size_t y_origin, size_t x_size, size_t y_size,
               Bool bord_top, Bool bord_bottom, Bool bord_left, Bool bord_right)
{
    // First fill in the rectangular area...
    x_origin *= nedsim->cell_size;
    x_origin += nedsim->origin_x_offset;
    y_origin *= nedsim->cell_size;
    y_origin += nedsim->origin_y_offset;
    x_size *= nedsim->cell_size;
    y_size *= nedsim->cell_size;
    set_color(nedsim, color);
    XFillRectangle(nedsim->dpy, nedsim->panel, nedsim->gc,
            x_origin, y_origin, x_size, y_size
    );

    // ...then give it a border, if requested.
    set_color(nedsim, COLOR(border));
    if (bord_top) {
        XFillRectangle(nedsim->dpy, nedsim->panel, nedsim->gc,
                x_origin, y_origin,
                x_size, nedsim->border_size
        );
    }
    if (bord_bottom) {
        XFillRectangle(nedsim->dpy, nedsim->panel, nedsim->gc,
                x_origin, (y_origin + y_size - nedsim->border_size),
                x_size, nedsim->border_size
        );
    }
    if (bord_left) {
        XFillRectangle(nedsim->dpy, nedsim->panel, nedsim->gc,
                x_origin, y_origin,
                nedsim->border_size, y_size
        );
    }
    if (bord_right) {
        XFillRectangle(nedsim->dpy, nedsim->panel, nedsim->gc,
                (x_origin + x_size - nedsim->border_size), y_origin,
                nedsim->border_size, y_size
        );
    }
}

// Draws filled circle in a square area with upper left corner at x,y.  When
// specifying the location and size to draw, all values are in units of
// 'cells'.
static void
draw_circular_area(struct NEDsim * nedsim, size_t x, size_t y, double diameter)
{
    // First, convert the function argument units from 'cells' to 'pixels'
    x *= nedsim->cell_size;
    y *= nedsim->cell_size;
    diameter *= nedsim->cell_size;

    // Add the panel's absolute x,y offset to the requested coordinates.
    x += nedsim->origin_x_offset;
    y += nedsim->origin_y_offset;

    // Shrink the circle to be a bit smaller than the bounding box allows. Note
    // that the three adjustment values must sum to 1.0.
    // For example, 0.7 + 0.15 + 0.15 = 1.0.
    x += (0.15 * diameter);
    y += (0.15 * diameter);
    diameter *= 0.7;

    // Because we only draw the bottom border on repeated rows (e.g. draw_wordline()),
    // we need to offset vertically by half a border height.
    y -= (0.5 * nedsim->border_size);

    // Start angle 0 and ending angle 360*64 is one full circle in Xlib units.
    XFillArc(nedsim->dpy, nedsim->panel, nedsim->gc, x, y, diameter, diameter, 0, 360*64);
}

// Draws text in a square area with upper left corner at (x_origin,y_origin).
// Requires that set_font_size() has been run at least once. All values are in
// units of 'cells'.
static void
draw_text(struct NEDsim * nedsim, const char * text,
        int x_origin, int y_origin, int x_size, int y_size, Bool horizontally_center)
{
    set_color(nedsim, COLOR(text));

    int text_x_size, text_y_size;
    get_text_size(nedsim, text, &text_x_size, &text_y_size);

    int local_y_offset = ((y_size * nedsim->cell_size) - text_y_size) / 2;
    int local_x_offset = 0;
    if (horizontally_center) local_x_offset = ((x_size * nedsim->cell_size) - text_x_size) / 2;

    XDrawString(nedsim->dpy, nedsim->panel, nedsim->gc,
            (x_origin * nedsim->cell_size + nedsim->origin_x_offset + local_x_offset),
            ((y_origin + y_size) * nedsim->cell_size + nedsim->origin_y_offset - local_y_offset),
            text, strlen(text)
    );
}

/* -------------------------------------------------------------------------- */
/* Dynamic Panel Functions                                                    */
/* -------------------------------------------------------------------------- */

// Draw the 32 lights corresponding to 'word' at coordinates ('x','y').
// Note that this function ONLY draws the lights and is used each frame for
// updating the panel. To draw the wordline area itself, use draw_wordline().
static void
draw_wordline_lights(struct NEDsim * nedsim, uint32_t word, int x, int y)
{
    for (int i = 0; i < WORDLINE_WIDTH; i++) {
        if (word & (1<<(WORDLINE_WIDTH-1-i))) {
            set_color(nedsim, COLOR(light_on));
        } else {
            set_color(nedsim, COLOR(light_off));
        }
        draw_circular_area(nedsim, x+i, y, WORDLINE_HEIGHT);
    }
}

// After the static front panel has been drawn at least once, this function
// updates all the dynamic parts of the panel (lights + text values) and is
// called every frame.
static void
update_display(struct NEDsim * nedsim)
{
    // Draw the halt indicator.
    if (nedsim->nedstate->halted) {
        set_color(nedsim, COLOR(error_on));
    } else {
        set_color(nedsim, COLOR(error_off));
    }
    draw_circular_area(nedsim, HALT_X_OFFSET, HALT_Y_OFFSET, HALT_WIDTH);

    // Draw the PSW "N" light.
    if (nedsim->nedstate->active_thread->psw->negative) {
        set_color(nedsim, COLOR(light_on));
    } else {
        set_color(nedsim, COLOR(light_off));
    }
    draw_circular_area(nedsim,
            PSW_N_X_OFFSET+1, PSW_Y_OFFSET+PSW_LABEL_HEIGHT, PSW_LIGHT_HEIGHT
    );

    // Draw the PSW "Z" light.
    if (nedsim->nedstate->active_thread->psw->zero) {
        set_color(nedsim, COLOR(light_on));
    } else {
        set_color(nedsim, COLOR(light_off));
    }
    draw_circular_area(nedsim,
            PSW_Z_X_OFFSET+1, PSW_Y_OFFSET+PSW_LABEL_HEIGHT, PSW_LIGHT_HEIGHT
    );


    // Draw the SC.
    for (int i = 0; i < SC_WIDTH; i++) {
        if ((SC_WIDTH-1-i) == nedsim->nedstate->active_thread->sc) {
            set_color(nedsim, COLOR(light_on));
        } else {
            set_color(nedsim, COLOR(light_off));
        }
        draw_circular_area(nedsim,
                SC_X_OFFSET+i, SC_Y_OFFSET+SC_LABEL_HEIGHT, SC_LIGHT_HEIGHT
        );
    }

    // Draw the PC.
    draw_wordline_lights(nedsim, nedsim->nedstate->active_thread->pc,
            PC_X_OFFSET, PC_Y_OFFSET+PC_LABEL_HEIGHT
    );

    // Draw the stack lights.
    int64_t top_of_stack = ((int64_t)nedsim->nedstate->active_thread->sp) - 1;
    for (int i = 0; i < nedsim->num_data_rows; i++) {
        if ((top_of_stack-i) >= 0) {
            draw_wordline_lights(nedsim,
                    nedsim->nedstate->active_thread->stack[top_of_stack-i],
                    STACK_X_OFFSET, STACK_Y_OFFSET+STACK_LABEL_HEIGHT+i
            );
        } else {
            draw_wordline_lights(nedsim, 0,
                    STACK_X_OFFSET, STACK_Y_OFFSET+STACK_LABEL_HEIGHT+i
            );
        }
    }

    // Draw the stack size in text.
    draw_rect_area(nedsim, COLOR(tertiary),
            STACK_X_OFFSET+(STACK_WIDTH/2), STACK_Y_OFFSET,
            STACK_WIDTH/2, STACK_LABEL_HEIGHT,
            True, True, False, False
    );
    char stack_size[11];
    snprintf(stack_size, sizeof(stack_size),"0x%08X", nedsim->nedstate->active_thread->sp);
    draw_text(nedsim, stack_size,
            (STACK_X_OFFSET+(STACK_WIDTH/2)+1), STACK_Y_OFFSET,
            STACK_WIDTH, STACK_LABEL_HEIGHT, False
    );

    // Draw the heap lights.
    for (int i = 0; i < nedsim->num_data_rows; i++) {
        draw_wordline_lights(nedsim,
                ram_r_word(nedsim->nedstate, nedsim->heap_window_offset+(i*BPW)),
                HEAP_X_OFFSET, HEAP_Y_OFFSET+HEAP_LABEL_HEIGHT+i
        );
    }
}

/* -------------------------------------------------------------------------- */
/* Static Panel Functions                                                     */
/* -------------------------------------------------------------------------- */

// Draws the panel itself. Not the lights/labels/etc, but the flat sheet of
// metal that is the front panel.
static void
draw_panel(struct NEDsim * nedsim)
{
    size_t overall_height_in_cells = HEADER_HEIGHT_IN_CELLS + nedsim->num_data_rows + FOOTER_HEIGHT_IN_CELLS;

    // Draw background color over entire window.
    set_color(nedsim, COLOR(panel_bg));
    XFillRectangle(nedsim->dpy, nedsim->panel, nedsim->gc, 0, 0, nedsim->dpy_width, nedsim->dpy_height);

    // Draw NED panel in foreground color.
    draw_rect_area(nedsim, COLOR(panel_fg),
            0, 0,
            OVERALL_WIDTH_IN_CELLS, overall_height_in_cells,
            False, False, False, False
    );

    // Give the panel rounded corners by first deleting the four right-angle corners...
    draw_rect_area(nedsim, COLOR(panel_bg),
            0, 0,
            1, 1,
            False, False, False, False
    );
    draw_rect_area(nedsim, COLOR(panel_bg),
            OVERALL_WIDTH_IN_CELLS-1, 0,
            1, 1,
            False, False, False, False
    );
    draw_rect_area(nedsim, COLOR(panel_bg),
            0, overall_height_in_cells-1,
            1, 1,
            False, False, False, False
    );
    draw_rect_area(nedsim, COLOR(panel_bg),
            OVERALL_WIDTH_IN_CELLS-1, overall_height_in_cells-1,
            1, 1,
            False, False, False, False
    );
    // ...and then replacing them with filled arcs, forming rounded corners.
    // Note that we use XFillArc() instead of draw_circular_area() since we do
    // not desire any padding.
    set_color(nedsim, COLOR(panel_fg));
    XFillArc(nedsim->dpy, nedsim->panel, nedsim->gc,
            nedsim->origin_x_offset,
            nedsim->origin_y_offset,
            nedsim->cell_size * 2, nedsim->cell_size * 2,
            180*64, -90*64
    );
    XFillArc(nedsim->dpy, nedsim->panel, nedsim->gc,
            nedsim->origin_x_offset + (nedsim->cell_size * (OVERALL_WIDTH_IN_CELLS-2)),
            nedsim->origin_y_offset,
            nedsim->cell_size * 2, nedsim->cell_size * 2,
            0, 90*64
    );
    XFillArc(nedsim->dpy, nedsim->panel, nedsim->gc,
            nedsim->origin_x_offset,
            nedsim->origin_y_offset + (nedsim->cell_size * (overall_height_in_cells-2)),
            nedsim->cell_size * 2, nedsim->cell_size * 2,
            180*64, 90*64
    );
    XFillArc(nedsim->dpy, nedsim->panel, nedsim->gc,
            nedsim->origin_x_offset + (nedsim->cell_size * (OVERALL_WIDTH_IN_CELLS-2)),
            nedsim->origin_y_offset + (nedsim->cell_size * (overall_height_in_cells-2)),
            nedsim->cell_size * 2, nedsim->cell_size * 2,
            0, -90*64
    );
}

static void
draw_logo(struct NEDsim * nedsim)
{
    draw_rect_area(nedsim, COLOR(primary),
            LOGO_X_OFFSET, LOGO_Y_OFFSET,
            LOGO_WIDTH, LOGO_NAME_HEIGHT,
            True, True, False, False
    );
    draw_rect_area(nedsim, COLOR(tertiary),
            LOGO_X_OFFSET, LOGO_Y_OFFSET+LOGO_NAME_HEIGHT,
            LOGO_WIDTH, LOGO_WEBSITE_HEIGHT,
            False, True, False, False
    );
    set_font_size(nedsim, LOGO_NAME_HEIGHT);
    draw_text(nedsim, "NED", LOGO_X_OFFSET, LOGO_Y_OFFSET,
            LOGO_WIDTH, LOGO_NAME_HEIGHT, True
    );
    set_font_size(nedsim, LOGO_WEBSITE_HEIGHT);
    draw_text(nedsim, "subgeniuskitty.com", LOGO_X_OFFSET, LOGO_Y_OFFSET+LOGO_NAME_HEIGHT,
            LOGO_WIDTH, LOGO_WEBSITE_HEIGHT, True
    );
}

static void
draw_halt(struct NEDsim * nedsim)
{
    draw_rect_area(nedsim, COLOR(tertiary),
            HALT_X_OFFSET, HALT_Y_OFFSET,
            HALT_WIDTH, HALT_LIGHT_HEIGHT,
            True, True, False, False
    );
    draw_rect_area(nedsim, COLOR(secondary),
            HALT_X_OFFSET, HALT_Y_OFFSET+HALT_LIGHT_HEIGHT,
            HALT_WIDTH, HALT_LABEL_HEIGHT,
            False, True, False, False
    );
    draw_text(nedsim, "HALT", HALT_X_OFFSET, HALT_Y_OFFSET+HALT_LIGHT_HEIGHT,
            HALT_WIDTH, HALT_LABEL_HEIGHT, True
    );
}

// Draw a single 32-bit NED word line at coordinates ('x','y').  To update with
// a specific value, call draw_wordline_lights() after drawing the wordline
// area at least once.
static void
draw_wordline(struct NEDsim * nedsim, int x, int y)
{
    // First, draw a solid box in the primary color over the entire wordline area.
    draw_rect_area(nedsim, COLOR(primary),
            x, y,
            WORDLINE_WIDTH, WORDLINE_HEIGHT,
            False, True, False, False
    );

    // Now, draw stripes in the secondary color.
    for (int i = 0; i < (WORDLINE_WIDTH/(2*WORDLINE_BITS_PER_STRIPE)); i++) {
        draw_rect_area(nedsim, COLOR(secondary),
                (x+(i*(WORDLINE_WIDTH/WORDLINE_BITS_PER_STRIPE))), y,
                WORDLINE_BITS_PER_STRIPE, WORDLINE_HEIGHT,
                False, True, False, False
        );
    }
}

static void
draw_pc(struct NEDsim * nedsim)
{
    draw_rect_area(nedsim, COLOR(tertiary),
            PC_X_OFFSET, PC_Y_OFFSET,
            PC_WIDTH, PC_LABEL_HEIGHT,
            True, True, False, False
    );
    draw_wordline(nedsim, PC_X_OFFSET, PC_Y_OFFSET+PC_LABEL_HEIGHT);
    draw_text(nedsim, "PC", PC_X_OFFSET, PC_Y_OFFSET, PC_WIDTH, PC_LABEL_HEIGHT, True);
}

static void
draw_sc(struct NEDsim * nedsim)
{
    draw_rect_area(nedsim, COLOR(secondary),
            SC_X_OFFSET, SC_Y_OFFSET,
            SC_WIDTH, SC_LABEL_HEIGHT,
            True, True, False, False
    );
    draw_rect_area(nedsim, COLOR(tertiary),
            SC_X_OFFSET, SC_Y_OFFSET+SC_LABEL_HEIGHT,
            SC_WIDTH, SC_LIGHT_HEIGHT,
            False, True, False, False
    );
    draw_text(nedsim, "SC", SC_X_OFFSET, SC_Y_OFFSET, SC_WIDTH, SC_LABEL_HEIGHT, True);
}

static void
draw_psw(struct NEDsim * nedsim)
{
    draw_rect_area(nedsim, COLOR(secondary),
            PSW_N_X_OFFSET, PSW_Y_OFFSET,
            PSW_LABEL_WIDTH, PSW_LABEL_HEIGHT,
            True, True, False, False
    );
    draw_rect_area(nedsim, COLOR(secondary),
            PSW_Z_X_OFFSET, PSW_Y_OFFSET,
            PSW_LABEL_WIDTH, PSW_LABEL_HEIGHT,
            True, True, False, False
    );
    draw_rect_area(nedsim, COLOR(tertiary),
            (PSW_N_X_OFFSET + 1), PSW_Y_OFFSET+PSW_LABEL_HEIGHT,
            PSW_LIGHT_WIDTH, PSW_LIGHT_HEIGHT,
            False, True, False, False
    );
    draw_rect_area(nedsim, COLOR(tertiary),
            (PSW_Z_X_OFFSET + 1), PSW_Y_OFFSET+PSW_LABEL_HEIGHT,
            PSW_LIGHT_WIDTH, PSW_LIGHT_HEIGHT,
            False, True, False, False
    );
    draw_text(nedsim, "N", PSW_N_X_OFFSET, PSW_Y_OFFSET,
            PSW_LABEL_WIDTH, PSW_LABEL_HEIGHT, True
    );
    draw_text(nedsim, "Z", PSW_Z_X_OFFSET, PSW_Y_OFFSET,
            PSW_LABEL_WIDTH, PSW_LABEL_HEIGHT, True
    );
}

static void
draw_stack(struct NEDsim * nedsim)
{
    draw_rect_area(nedsim, COLOR(tertiary),
            STACK_X_OFFSET, STACK_Y_OFFSET,
            STACK_WIDTH, STACK_LABEL_HEIGHT,
            True, True, False, False
    );
    for (int i = 0; i < nedsim->num_data_rows; i++) {
        draw_wordline(nedsim, STACK_X_OFFSET, STACK_Y_OFFSET+STACK_LABEL_HEIGHT+i);
    }
    draw_text(nedsim, "Stack Size:", STACK_X_OFFSET+1, STACK_Y_OFFSET,
            STACK_WIDTH, STACK_LABEL_HEIGHT, False
    );
}

static void
draw_heap(struct NEDsim * nedsim)
{
    draw_rect_area(nedsim, COLOR(tertiary),
            HEAP_X_OFFSET, HEAP_Y_OFFSET,
            HEAP_WIDTH, HEAP_LABEL_HEIGHT,
            True, True, False, False
    );
    for (int i = 0; i < nedsim->num_data_rows; i++) {
        draw_wordline(nedsim, HEAP_X_OFFSET, HEAP_Y_OFFSET+HEAP_LABEL_HEIGHT+i);
    }
    draw_text(nedsim, "RAM Base:", HEAP_X_OFFSET+1, HEAP_Y_OFFSET,
            HEAP_WIDTH, HEAP_LABEL_HEIGHT, False
    );
    char address[11];
    snprintf(address, sizeof(address), "0x%08X", nedsim->heap_window_offset);
    draw_text(nedsim, address, (HEAP_X_OFFSET+(HEAP_WIDTH/2)+1), HEAP_Y_OFFSET,
            HEAP_WIDTH, HEAP_LABEL_HEIGHT, False
    );
}

/* -------------------------------------------------------------------------- */
/* Screenhack API Functions                                                   */
/* -------------------------------------------------------------------------- */

static Bool
NEDsim_event(Display * dpy, Window win, void * closure, XEvent * event)
{
    return False;
}

static void
NEDsim_free(Display * dpy, Window win, void * closure)
{
    struct NEDsim * nedsim = closure;

    if (nedsim->nedstate != NULL) {
        free(nedsim->nedstate->active_thread->psw);
        free(nedsim->nedstate->active_thread);
        free(nedsim->nedstate->hack);
        free(nedsim->nedstate);
    }

    XFreeGC(nedsim->dpy, nedsim->gc);
    XFreePixmap(nedsim->dpy, nedsim->panel);
    free(nedsim);
}

static void *
NEDsim_init(Display * dpy, Window win)
{
    // Basic Setup ---------------------------------------------------------------------------------

    struct NEDsim * nedsim;

    nedsim = calloc(1, sizeof(*nedsim));
    if (!nedsim) {
        fprintf(stderr, "ERROR: Failed to calloc() for NEDsim struct in NEDsim_init().\n");
        exit(EXIT_FAILURE);
    }

    nedsim->dpy = dpy;
    nedsim->win = win;

    XGCValues gcv;
    nedsim->gc = XCreateGC(nedsim->dpy, nedsim->win, GCForeground, &gcv);

    XWindowAttributes xgwa;
    XGetWindowAttributes(nedsim->dpy, nedsim->win, &xgwa);
    nedsim->dpy_width = xgwa.width;
    nedsim->dpy_height = xgwa.height;

    nedsim->panel = XCreatePixmap(nedsim->dpy, nedsim->win, nedsim->dpy_width, nedsim->dpy_height, xgwa.depth);

    // Process User Requests -----------------------------------------------------------------------

    nedsim->delay = get_integer_resource(nedsim->dpy, "delay", "Integer");
    nedsim->delay *= 1000; /* Turn milliseconds into microseconds. */

    // If the user did not supply a program for execution, randomly select one
    // of the included programs.
    char * input_file = get_string_resource(nedsim->dpy, "binary", "String");
    if (input_file == NULL) {
        size_t id = random() % sizeof(ned_programs)/sizeof(ned_programs[0]);
        nedsim->nedstate = init_simulator(NULL, ned_programs[id].binary_blob, ned_programs[id].blob_size);
        nedsim->heap_window_offset = ned_programs[id].heap_window_offset;
    } else {
        nedsim->nedstate = init_simulator(input_file, NULL, 0);
        // Note: Since we're accepting this as a runtime CLI option, we really
        // should verify that `heap_window_offset+num_data_rows` doesn't exceed
        // the limits of NED's simulated RAM. However, since the RAM size is a
        // #define located in the simulator itself, and since I mostly plan to
        // use this screensaver with built-in programs, for now we'll just let
        // it segfault if the user requests a window outside simulated RAM.
        nedsim->heap_window_offset = get_integer_resource(nedsim->dpy, "heapwindow", "Integer");
    }

    // If the user did not select a color scheme, select one randomly.
    nedsim->color_index = get_integer_resource(nedsim->dpy, "color", "Integer");
    if (nedsim->color_index == -1) {
        nedsim->color_index = random() % sizeof(color_list)/sizeof(color_list[0]);
    } else if (nedsim->color_index >= sizeof(color_list)/sizeof(color_list[0])) {
        fprintf(stderr, "WARNING: Color index out of range.\n");
        nedsim->color_index = 0;
    }

    // Scale Panel To Screen -----------------------------------------------------------------------

    // We desire to make the panel as wide as possible while also making the
    // cell size an integer pixel size.
    nedsim->cell_size = nedsim->dpy_width / OVERALL_WIDTH_IN_CELLS;
    // Cell sizes below about 10 pixels result in unreadable fonts. For now,
    // we'll use that as our cutoff. It also works well with a 1:10 border:cell
    // ratio.
    if (nedsim->cell_size < 10) {
        nedsim->suitable_display = False;
        return nedsim;
    }
    // Center the panel horizontally.
    nedsim->origin_x_offset = (nedsim->dpy_width -
            (nedsim->cell_size * OVERALL_WIDTH_IN_CELLS)) / 2;

    // The stack and heap displays are variable height. Size them to fit the
    // display window.
    int available_space_for_data_rows = nedsim->dpy_height -
            (nedsim->cell_size * (HEADER_HEIGHT_IN_CELLS + FOOTER_HEIGHT_IN_CELLS));
    for (int i = 0; ; i++) {
        if ((nedsim->cell_size * i) > available_space_for_data_rows) {
            nedsim->num_data_rows = --i;
            break;
        }
    }
    // Enforce a minimum vertical size, though '4' is arbitrary.
    if (nedsim->num_data_rows < 4) {
        nedsim->suitable_display = False;
        return nedsim;
    }
    // Center the panel vertically.
    nedsim->origin_y_offset = (nedsim->dpy_height -
            (nedsim->cell_size * (HEADER_HEIGHT_IN_CELLS + nedsim->num_data_rows + FOOTER_HEIGHT_IN_CELLS))) / 2;

    // Scale border relative to cell_size in a 1:10 relationship.
    nedsim->border_size = nedsim->cell_size / 10;

    // Draw Initial Panel --------------------------------------------------------------------------

    draw_panel(nedsim);
    draw_logo(nedsim);
    draw_halt(nedsim);
    draw_pc(nedsim);
    draw_sc(nedsim);
    draw_psw(nedsim);
    draw_stack(nedsim);
    draw_heap(nedsim);

    nedsim->suitable_display = True;

    return nedsim;
}

static unsigned long
NEDsim_draw(Display * dpy, Window win, void * closure)
{
    struct NEDsim * nedsim = closure;

    // Update the panel display buffer.
    if (nedsim->suitable_display) {
        nedsim->nedstate = run_simulator(nedsim->nedstate);
        update_display(nedsim);
    } else {
        set_color(nedsim, COLOR(error_on));
        XFillRectangle(nedsim->dpy, nedsim->panel, nedsim->gc, 0, 0, nedsim->dpy_width, nedsim->dpy_height);
    }

    // Copy panel buffer to display window.
    XCopyArea(nedsim->dpy, nedsim->panel, nedsim->win, nedsim->gc, 0, 0, nedsim->dpy_width, nedsim->dpy_height, 0, 0);

    return nedsim->delay;
}

static void
NEDsim_reshape(Display * dpy, Window win, void * closure, unsigned int w, unsigned int h)
{
    struct NEDsim * nedsim = closure;

    // Only re-initialize the display if it changed size.
    XWindowAttributes xgwa;
    XGetWindowAttributes(nedsim->dpy, nedsim->win, &xgwa);
    if (nedsim->dpy_width != xgwa.width || nedsim->dpy_height != xgwa.height) {
        // Although we are re-initializing the display, we wish to retain the
        // in-progress NED simulation. Thus, we retain the NEDstate pointer.
        struct NEDstate * original_nedstate = nedsim->nedstate;
        nedsim->nedstate = NULL;
        NEDsim_free(dpy, win, nedsim);
        struct NEDsim * new_nedsim = NEDsim_init(dpy, win);
        new_nedsim->nedstate = original_nedstate;
        closure = new_nedsim;
    }
}

static const char * NEDsim_defaults[] = {
    "*delay:              250",
    "*color:              -1",
    "*heapwindow:         0x20000000",
    0
};

static XrmOptionDescRec NEDsim_options[] = {
    { "-delay",              ".delay",                  XrmoptionSepArg, 0      },
    { "-binary",             ".binary",                 XrmoptionSepArg, 0      },
    { "-color",              ".color",                  XrmoptionSepArg, 0      },
    { "-heapwindow",         ".heapwindow",             XrmoptionSepArg, 0      },
    { 0, 0, 0, 0 }
};

XSCREENSAVER_MODULE ("Blinken-lights simulator for NED1 CPU architecture.", NEDsim)