jbroadwell tried to simulate an hd44780 lcd in http://www.microchip.com/forums/m227487.aspx This is the indented code posted therein.

hd44780sim.serialwombat.c

/*
 * hd44780 simulator
 * from
 * http://www.microchip.com/forums/m227487-print.aspx
 * 2/2007 by jbroadwell
 */
#define FIRST_NIBBLE 0
#define SECOND_NIBBLE 1
#define DDRAM_SIZE 128
typedef struct hd44780_n
{
    uint8 Epin;
    uint8 RSpin;
    uint8 datapin[4];
    uint8 memory[DDRAM_SIZE];
    uint8 lines;
    uint8 width;
    uint8 linestart[4];
    uint8 rxstate;
    uint8 E_last_state;
    uint8 display_on;
    uint8 data_in;
    int16 cursorpos;
    int8 autoincdir;
    uint8 cursor_on;
    uint8 cursor_blink;
    uint8 interface_length;
    uint8 interface_lines;
    uint8 interface_characters;
} hd44780_t;

hd44780_t hd44780_display[4];

#define h hd44780_display[instance]

void init_hd44780(uint8 instance, uint8 Epin, uint8 RSpin, uint8 datapin7, uint8 datapin6, uint8 datapin5, uint8 datapin4, uint8 lines, uint8 width, uint8 linestart0, uint8 linestart1, uint8 linestart2, uint8 linestart3)
{
    int i;
    h.Epin = Epin;
    h.RSpin = RSpin;
    h.datapin[0] = datapin7;
    h.datapin[1] = datapin6;
    h.datapin[2] = datapin5;
    h.datapin[3] = datapin4;
    hd44780_display[instance].lines = lines;
    h.width = width;
    h.linestart[0] = linestart0;
    h.linestart[1] = linestart1;
    h.linestart[2] = linestart2;
    h.linestart[3] = linestart3;
    for (i = 0; i < DDRAM_SIZE; ++i)
    {
        h.memory[i] = '}';
    }
    h.E_last_state = LOW;
    h.display_on = 0;

}

void print_hd44780(uint8 instance, uint8 top, uint8 bottom)
{
    int line, i;
    if (h.display_on)
    {
        if (top)
        {
            printf("  ");
            for (i = 0; i < h.width; ++i)
            {
                printf("%1d", i % 10);
            }
            printf("n");
            printf("+=");
            for (i = 0; i < h.width; ++i)
            {
                printf("=");
            }
            printf("=+n");
            printf("| ");
            for (i = 0; i < h.width; ++i)
            {
                printf(" ");
            }
            printf(" |n");
        }
        for (line = 0; line < h.lines; ++line)
        {
            printf("| ");
            for (i = 0; i < h.width; ++i)
            {
                printf("%c", h.memory[i + h.linestart[line]]);
            }
            printf(" |n");
        }
        if (bottom)
        {
            printf("| ");
            for (i = 0; i < h.width; ++i)
            {
                printf(" ");
            }
            printf(" |n");
            printf("+=");
            for (i = 0; i < h.width; ++i)
            {
                printf("=");
            }
            printf("=+n");
        }
    }
    else
    {
        printf("nnDISPLAY IS TURNED OFFn");
    }
}

void hd44780_update(uint8 instance)
{
    int i;
    if (h.E_last_state == 1 && read_pin(hd44780_display[instance].Epin) == 0)
    {
        if (h.rxstate == FIRST_NIBBLE)
        {
            h.data_in = (read_pin(h.datapin[0]) > 0);
            h.data_in <<= 1;
            h.data_in += (read_pin(h.datapin[1]) > 0);
            h.data_in <<= 1;
            h.data_in += (read_pin(h.datapin[2]) > 0);
            h.data_in <<= 1;
            h.data_in += (read_pin(h.datapin[3]) > 0);
            h.rxstate = SECOND_NIBBLE;
        }
        else
        {
            h.data_in <<= 1;
            h.data_in += (read_pin(h.datapin[0]) > 0);
            h.data_in <<= 1;
            h.data_in += (read_pin(h.datapin[1]) > 0);
            h.data_in <<= 1;
            h.data_in += (read_pin(h.datapin[2]) > 0);
            h.data_in <<= 1;
            h.data_in += (read_pin(h.datapin[3]) > 0);
            h.rxstate = FIRST_NIBBLE;
            //printf ("Processing data 0x%02X, RS pin is %dn",h.data_in,read_pin(h.RSpin));
            if (read_pin(h.RSpin))
            {
                //Data Entry
                //printf ("Processing data entryn");
                h.memory[h.cursorpos] = h.data_in;
                h.cursorpos += h.autoincdir;
                if (h.cursorpos >= DDRAM_SIZE)
                {
                    h.cursorpos = 0;
                }
                if (h.cursorpos < 0)
                {
                    h.cursorpos = DDRAM_SIZE - 1;
                }


            }
            else
            {
                //command entry
                if (h.data_in == 1)
                {
                    //printf ("Processing clear displayn");
                    //Clear display
                    for (i = 0; i < 80; ++i)
                    {
                        h.memory[i] = ' ';
                    }
                    h.cursorpos = 0;
                }
                else if ((h.data_in & 0xFE) == 0x02)
                {
                    //printf("Processing cursor homen");
                    h.cursorpos = 0;
                }
                else if ((h.data_in & 0xFC) == 0x04)
                {
                    //printf ("Processing Entry mode Setn");
                    if (h.data_in & 0x02)
                    {
                        //printf("Shift Cursorn");
                        if (h.data_in & 0x01)
                        {
                            h.autoincdir = -1;
                        }
                        else
                        {
                            h.autoincdir = 1;
                        }
                    }
                    else
                    {
                        h.autoincdir = 0;
                    }
                }
                else if ((h.data_in & 0xF8) == 0x08)
                {
                    //Display on/off control
                    if (h.data_in & 0x04)
                    {
                        h.display_on = 1;
                    }
                    else
                    {
                        h.display_on = 0;
                    }
                    if (h.data_in & 0x02)
                    {
                        h.cursor_on = 1;
                    }
                    else
                    {
                        h.cursor_on = 0;
                    }
                    if (h.data_in & 0x01)
                    {
                        h.cursor_blink = 1;
                    }
                    else
                    {
                        h.cursor_blink = 0;
                    }
                }
                else if ((h.data_in & 0xE0) == 0x02)
                {
                    //  INTERFACE control
                    if (h.data_in & 0x08)
                    {
                        h.interface_length = 8;
                    }
                    else
                    {
                        h.interface_length = 4;
                    }
                    if (h.data_in & 0x04)
                    {
                        h.interface_lines = 2;
                    }
                    else
                    {
                        h.interface_lines = 1;
                    }
                    if (h.data_in & 0x02)
                    {
                        h.interface_characters = 1;
                    }
                    else
                    {
                        h.interface_characters = 0;
                    }
                }
                else if (h.data_in & 0x80)
                {
                    // Set DDRAM address
                    //printf ("Setting DDRAM address to 0x%02Xn", (h.data_in & 0x7F));
                    h.cursorpos = (h.data_in & 0x7F);
                }
            }
        }
    }
    hd44780_display[instance].E_last_state = read_pin(hd44780_display[instance].Epin);
}