diff --git a/Makefile b/Makefile
index 2633c8d..91303b0 100644
--- a/Makefile
+++ b/Makefile
@@ -11,8 +11,8 @@ MCU=attiny2313
 AVRDUDE_MCU=attiny2313
 
 AVRDUDE_FUSES=lfuse:w:0xff:m hfuse:w:0xdb:m efuse:w:0xff:m
-#AVRDUDE_PROG := -c avr910 -b 115200 -P /dev/ispprog
-AVRDUDE_PROG := -c dragon_isp -P usb
+AVRDUDE_PROG := -c avr910 -b 115200 -P /dev/ispprog
+#AVRDUDE_PROG := -c dragon_isp -P usb
 
 # ---------------------------------------------------------------------------
 
diff --git a/main.c b/main.c
index b0c96ac..f01fe13 100644
--- a/main.c
+++ b/main.c
@@ -65,11 +65,62 @@
 
 #define UART_CALC_BAUDRATE(baudRate) (((uint32_t)F_CPU) / (((uint32_t)baudRate)*16) -1)
 
+/* ************************************************************************* */
+
+#define MSGTYPE_VERSION_REQ     0x01    /* no payload */
+#define MSGTYPE_PAGESIZE_REQ    0x02    /* no payload */
+#define MSGTYPE_CONFIG_REQ      0x03    /* eprom_type(1), pagesize(1), reset_polarity(1) */
+#define MSGTYPE_PROGMODE_REQ    0x04    /* progmode(1) */
+#define MSGTYPE_SETADDRESS_REQ  0x05    /* address(3) msb first */
+#define MSGTYPE_WRITE_REQ       0x06    /* data(0-pagesize) */
+#define MSGTYPE_READ_REQ        0x07    /* length(1) */
+
+#define MSGTYPE_ERROR_RSP       0x80    /* error_code(1) */
+#define MSGTYPE_VERSION_RSP     0x81    /* version(?) */
+#define MSGTYPE_PAGESIZE_RSP    0x82    /* pagesize(1) */
+#define MSGTYPE_CONFIG_RSP      0x83    /* no payload */
+#define MSGTYPE_PROGMODE_RSP    0x84    /* no payload */
+#define MSGTYPE_SETADDRESS_RSP  0x85    /* no payload */
+#define MSGTYPE_WRITE_RSP       0x86    /* no payload */
+#define MSGTYPE_READ_RSP        0x87    /* data(0-pagesize) */
+
+#define SUCCESS                 0x00
+#define ERROR_UNKNOWN_COMMAND   0x01    /* unknown message type */
+#define ERROR_NOT_SUPPORTED     0x02    /* command not supported */
+#define ERROR_INVALID_MODE      0x03    /* invalid progmode */
+#define ERROR_INVALID_PARAMETER 0x04    /* invalid parameter in request */
+#define ERROR_INVALID_ADDRESS   0x05    /* write outside of configured region */
+
+#define RESET_POLARITY_LOW      0x00    /* low active reset */
+#define RESET_POLARITY_HIGH     0x01    /* high active reset */
+
+#define EPROM_TYPE_2K           0x02    /* 2716 */
+#define EPROM_TYPE_4K           0x04    /* 2732 */
+#define EPROM_TYPE_8K           0x08    /* 2764 */
+#define EPROM_TYPE_16K          0x10    /* 27128 */
+#define EPROM_TYPE_32K          0x20    /* 27256 */
+#define EPROM_TYPE_64K          0x40    /* 27512 */
+#define EPROM_TYPE_128K         0x80    /* 27010 */
+
+#define PROGMODE_DISABLED       0x00    /* target running, no write access to RAM */
+#define PROGMODE_ENABLED        0x01    /* target reset, write access to RAM */
+
+#define PAGESIZE_MAX            128
+
+/* ************************************************************************* */
+
 struct _globdata {
-    uint8_t address_mask;
-    uint8_t reset_polarity;
+    uint32_t    address_max;
+    uint32_t    address;
+
+    uint8_t     progmode;
+    uint8_t     reset_polarity;
+    uint8_t     pagesize;
+
+    uint8_t     address_mask;
 };
 
+static const uint8_t version_str[] = "epromsim v1.00";
 static struct _globdata gdata = { 0 };
 
 
@@ -100,10 +151,14 @@ static void shift_data(uint8_t data)
 {
     uint8_t mask;
 
-    for (mask = 0x01; mask != 0; mask <<= 1) {
-        if (data & mask) {
+    for (mask = 0x01; mask != 0; mask <<= 1)
+    {
+        if (data & mask)
+        {
             PORTB |= (1<<SREG_DAT);
-        } else {
+        }
+        else
+        {
             PORTB &= ~(1<<SREG_DAT);
         }
 
@@ -126,32 +181,69 @@ static void store_pulse(void)
 
 
 /* *************************************************************************
- * write pulse for SRAM and increment counter
+ * write pulse for SRAM
  * ************************************************************************* */
-static void write_inc_pulse(void)
+static void write_pulse(void)
 {
     /* positive edge clocks in data */
     PORTB &= ~(1<<nRAM_WR);
     PORTB |= (1<<nRAM_WR);
+} /* write_pulse */
 
+
+/* *************************************************************************
+ * reset address counter
+ * ************************************************************************* */
+static void address_reset(void)
+{
+    PORTD &= ~(1<<nTARGET_EN);
+    PORTD |= (1<<nTARGET_EN);
+} /* address_reset */
+
+
+/* *************************************************************************
+ * increment address counter
+ * ************************************************************************* */
+static void address_inc_pulse(void)
+{
     /* positive edge increments counter */
     PORTB |= (1<<CNT_CLK);
     PORTB &= ~(1<<CNT_CLK);
-} /* write_inc_pulse */
+} /* address_inc_pulse */
+
+
+/* *************************************************************************
+ * controls A16 line
+ * ************************************************************************* */
+static void address_set_a16(uint8_t enable)
+{
+    if (enable)
+    {
+        PORTD |= (1<<EMU_A16);
+    }
+    else
+    {
+        PORTD &= ~(1<<EMU_A16);
+    }
+} /* address_set_a16 */
 
 
 /* *************************************************************************
  * switch access to RAM between emulator and target
  * ************************************************************************* */
-static void do_reset(uint8_t enable)
+static void set_progmode(uint8_t progmode)
 {
-    if (enable) {
+    gdata.progmode = progmode;
+
+    if (progmode)
+    {
         /* switch RAM access to EMU */
         PORTD |= (1<<nTARGET_EN);
         PORTD &= ~(1<<nLED_RT);
         PORTB &= ~(1<<nEMU_EN);
-
-    } else {
+    }
+    else
+    {
         /* set eprom address line mask */
         shift_data(gdata.address_mask);
         shift_data(0x00);
@@ -161,109 +253,136 @@ static void do_reset(uint8_t enable)
         PORTB |= (1<<nEMU_EN);
         PORTD &= ~(1<<nTARGET_EN);
         PORTD |= (1<<nLED_RT);
+
+        /* address counter is reset */
+        gdata.address = 0x00;
     }
 
     /* set RESET_OUT */
-    if (enable ^ gdata.reset_polarity) {
+    if (progmode ^ gdata.reset_polarity)
+    {
         PORTB &= ~(1<<RESET_OUT);
-    } else {
+    }
+    else
+    {
         PORTB |= (1<<RESET_OUT);
     }
-} /* do_reset */
+} /* set_progmode */
 
 
 /* *************************************************************************
- * configures number of addresslines and loads data from UART into RAM
+ * set_address
  * ************************************************************************* */
-static void do_load(uint8_t type, uint32_t length)
+static void set_address(uint32_t address)
 {
-    uint32_t size;
-    switch (type) {
-        case 0x02: /* 2716 - 2kB */
-            size = 0x0800;
-            gdata.address_mask = 0x00;
-            break;
-
-        case 0x04: /* 2732 - 4kB */
-            size = 0x1000;
-            gdata.address_mask = 0x80;
-            break;
-
-        case 0x08: /* 2764 - 8kB */
-            size = 0x2000;
-            gdata.address_mask = 0xC0;
-            break;
-
-        case 0x10: /* 27128 - 16kB */
-            size = 0x4000;
-            gdata.address_mask = 0xE0;
-            break;
-
-        case 0x20: /* 27256 - 32kB */
-            size = 0x8000;
-            gdata.address_mask = 0xF0;
-            break;
-
-        case 0x40: /* 27512 - 64kB */
-            size = 0x10000;
-            gdata.address_mask = 0xF8;
-            break;
-
-        default:
-        case 0x80: /* 27010 - 128kB */
-            size = 0x20000;
-            gdata.address_mask = 0xFC;
-            break;
+    /* reset address counter */
+    if (address < gdata.address)
+    {
+        address_reset();
+        gdata.address = 0;
     }
 
-    do_reset(1);
+    // TODO: A16 is not controlled by counter
 
-    uint32_t i;
-    uint8_t data;
-    for (i = 0; i < size; i++) {
-        if (i > 0xFFFF) {
-            PORTD |= (1<<EMU_A16);
-        } else {
-            PORTD &= ~(1<<EMU_A16);
-        }
-
-        data = (i < length) ? ser_recv() : 0xFF;
-        shift_data(data);
-        store_pulse();
-        write_inc_pulse();
+    while (gdata.address++ < address)
+    {
+        address_inc_pulse();
     }
-
-    do_reset(0);
-} /* do_load */
+} /* set_address */
 
 
 /* *************************************************************************
- * clears whole RAM (writes 0xFF)
+ * write_data
+ * ************************************************************************* */
+static void write_data(uint8_t length)
+{
+    while (length--)
+    {
+        address_set_a16(gdata.address++ > 0xFFFF);
+
+        shift_data(ser_recv());
+        store_pulse();
+        write_pulse();
+        address_inc_pulse();
+    }
+} /* write_data */
+
+
+/* *************************************************************************
+ * fill RAM with 0xFF
  * ************************************************************************* */
 static void do_clear(void)
 {
-    do_reset(1);
+    uint16_t i = 0xFFFF;
 
     shift_data(0xFF);
 
-    uint16_t i = 0xFFFF;
-
     PORTD &= ~(1<<EMU_A16);
     do {
         store_pulse();
-        write_inc_pulse();
+        write_pulse();
+        address_inc_pulse();
     } while (i--);
 
     PORTD |= (1<<EMU_A16);
     do {
         store_pulse();
-        write_inc_pulse();
+        write_pulse();
+        address_inc_pulse();
     } while (i--);
-
-    do_reset(0);
 } /* do_clear */
 
 
+/* *************************************************************************
+ * configures number of addresslines
+ * ************************************************************************* */
+static uint8_t set_eprom_type(uint8_t type)
+{
+    switch (type)
+    {
+        case EPROM_TYPE_2K:
+            gdata.address_max   = 0x0800;
+            gdata.address_mask  = 0x00;
+            break;
+
+        case EPROM_TYPE_4K:
+            gdata.address_max   = 0x1000;
+            gdata.address_mask  = 0x80;
+            break;
+
+        case EPROM_TYPE_8K:
+            gdata.address_max   = 0x2000;
+            gdata.address_mask  = 0xC0;
+            break;
+
+        case EPROM_TYPE_16K:
+            gdata.address_max   = 0x4000;
+            gdata.address_mask  = 0xE0;
+            break;
+
+        case EPROM_TYPE_32K:
+            gdata.address_max   = 0x8000;
+            gdata.address_mask  = 0xF0;
+            break;
+
+        case EPROM_TYPE_64K:
+            gdata.address_max   = 0x10000;
+            gdata.address_mask  = 0xF8;
+            break;
+
+        case EPROM_TYPE_128K:
+            gdata.address_max   = 0x20000;
+            gdata.address_mask  = 0xFC;
+            break;
+
+        default:
+            return ERROR_INVALID_PARAMETER;
+    }
+
+    return SUCCESS;
+} /* set_eprom_type */
+
+
 #if 0
 /* Powerfail / Wakeup */
 ISR(INT0_vect)
@@ -325,44 +444,164 @@ int main(void)
     PORTD &= ~((1<<nLED_GN) | (1<<nTARGET_EN));
 
     /* init RAM */
-    gdata.address_mask = 0x3F;
+    set_eprom_type(EPROM_TYPE_128K);
+    set_progmode(PROGMODE_ENABLED);
     do_clear();
+    set_progmode(PROGMODE_DISABLED);
 
-    while (1) {
-        uint8_t cmd = ser_recv();
+    while (1)
+    {
+        uint8_t msgtype = ser_recv();
+        uint8_t length = ser_recv();
+        uint8_t error_code = ERROR_INVALID_PARAMETER;
 
-        switch (cmd) {
-            case 'p': /* low active reset */
-                gdata.reset_polarity = 0;
-                ser_send('\n');
-                break;
-
-            case 'P': /* high active reset */
-                gdata.reset_polarity = 1;
-                ser_send('\n');
-                break;
-
-            case 'r': /* reset pulse */
-                do_reset(1);
-                _delay_ms(10);
-                do_reset(0);
-                ser_send('\n');
-                break;
-
-            case 'l': /* load eprom content */
+        switch (msgtype)
+        {
+            case MSGTYPE_VERSION_REQ:
+                if (length == 0x00)
                 {
-                    uint8_t  type   = ser_recv();
-                    uint32_t length = (uint32_t)ser_recv() << 16;
-                    length |= ser_recv() << 8;
-                    length |= ser_recv();
+                    ser_send(MSGTYPE_VERSION_RSP);
+                    ser_send(sizeof(version_str) -1);
 
-                    do_load(type, length);
+                    const uint8_t * p_data = version_str;
+                    while (*p_data)
+                    {
+                        ser_send(*p_data++);
+                    }
+
+                    error_code = SUCCESS;
                 }
                 break;
 
+            case MSGTYPE_PAGESIZE_REQ:
+                if (length == 0x00)
+                {
+                    ser_send(MSGTYPE_PAGESIZE_RSP);
+                    ser_send(0x01);
+                    ser_send(PAGESIZE_MAX);
+
+                    error_code = SUCCESS;
+                }
+                break;
+
+            case MSGTYPE_CONFIG_REQ:
+                if (length == 0x03)
+                {
+                    uint8_t eprom_type = ser_recv();
+                    uint8_t pagesize = ser_recv();
+                    uint8_t reset_polarity = ser_recv();
+
+                    if (gdata.progmode == PROGMODE_ENABLED)
+                    {
+                        error_code = ERROR_INVALID_MODE;
+                    }
+                    else
+                    {
+                        if ((reset_polarity <= RESET_POLARITY_HIGH) &&
+                            (pagesize <= PAGESIZE_MAX) &&
+                            (set_eprom_type(eprom_type) == SUCCESS)
+                           )
+                        {
+                            gdata.pagesize = pagesize;
+                            gdata.reset_polarity = reset_polarity;
+
+                            ser_send(MSGTYPE_CONFIG_RSP);
+                            ser_send(0x00);
+                            error_code = SUCCESS;
+                        }
+                    }
+                }
+                break;
+
+            case MSGTYPE_PROGMODE_REQ:
+                if (length == 0x01)
+                {
+                    uint8_t progmode = ser_recv();
+
+                    if (progmode <= PROGMODE_ENABLED)
+                    {
+                        set_progmode(progmode);
+
+                        ser_send(MSGTYPE_PROGMODE_RSP);
+                        ser_send(0x00);
+                        error_code = SUCCESS;
+                    }
+                }
+                break;
+
+            case MSGTYPE_SETADDRESS_REQ:
+                if (length == 0x03)
+                {
+                    uint32_t address;
+
+                    address = ser_recv();
+                    address = (address << 8) | ser_recv();
+                    address = (address << 8) | ser_recv();
+
+                    if (address < gdata.address_max)
+                    {
+                        if (gdata.progmode == PROGMODE_DISABLED)
+                        {
+                            error_code = ERROR_INVALID_MODE;
+                        }
+                        else
+                        {
+                            set_address(address);
+
+                            ser_send(MSGTYPE_SETADDRESS_RSP);
+                            ser_send(0x00);
+                            error_code = SUCCESS;
+                        }
+                    }
+                }
+                break;
+
+            case MSGTYPE_WRITE_REQ:
+                if ((length > 0) &&
+                    (length <= gdata.pagesize)
+                   )
+                {
+                    if ((gdata.address >= gdata.address_max) ||
+                        ((gdata.address + length) > gdata.address_max)
+                       )
+                    {
+                        error_code = ERROR_INVALID_ADDRESS;
+                    }
+                    else if (gdata.progmode == PROGMODE_DISABLED)
+                    {
+                        error_code = ERROR_INVALID_MODE;
+                    }
+                    else
+                    {
+                        write_data(length);
+
+                        ser_send(MSGTYPE_WRITE_RSP);
+                        ser_send(0x00);
+                        error_code = SUCCESS;
+                    }
+                }
+                break;
+
+            case MSGTYPE_READ_REQ:
+                error_code = ERROR_NOT_SUPPORTED;
+                break;
+
             default:
-                ser_send('?');
+                error_code = ERROR_UNKNOWN_COMMAND;
                 break;
         }
+
+        if (error_code != SUCCESS)
+        {
+            /* read remaining request */
+            while (length--)
+            {
+                (void)ser_recv();
+            }
+
+            ser_send(MSGTYPE_ERROR_RSP);
+            ser_send(0x01);
+            ser_send(error_code);
+        }
     }
-}
+} /* main */