diff --git a/src/mame/drivers/gnat10.cpp b/src/mame/drivers/gnat10.cpp
index 71281562694..821bb249a7b 100644
--- a/src/mame/drivers/gnat10.cpp
+++ b/src/mame/drivers/gnat10.cpp
@@ -10,11 +10,26 @@
 ****************************************************************************/
 
 #include "emu.h"
+//#include "bus/ieee488/ieee488.h"
+//#include "bus/rs232/rs232.h"
+//#include "machine/tms9914.h"
 #include "cpu/z80/z80.h"
+#include "cpu/i8085/i8085.h"
 #include "imagedev/floppy.h"
+//#include "machine/am9511.h"
+//#include "machine/am9517a.h"
+//#include "machine/i8155.h"
+//#include "machine/i8251.h"
 #include "machine/com8116.h"
+//#include "machine/mm58167.h"
+//#include "machine/tms9914.h"
 #include "machine/wd_fdc.h"
+//#include "machine/z80ctc.h"
+//#include "machine/z80pio.h"
 #include "machine/z80sio.h"
+//#include "video/mc6845.h"
+//#include "screen.h"
+//#include "speaker.h"
 
 class gnat10_state : public driver_device
 {
@@ -28,6 +43,7 @@ public:
 		, m_monitor(*this, "monitor")
 		, m_ram(*this, "ram")
 		, m_ram_on(false)
+		, m_prom_disable(false)
 	{
 	}
 
@@ -38,14 +54,17 @@ protected:
 	virtual void machine_reset() override;
 
 private:
-	u8 ram_r(offs_t offset);
+	u8 mem_r(offs_t offset);
 	u8 floppy_status_r();
 	void floppy_latch_w(u8 data);
+	u8 rtc_r(offs_t offset);
+	void rtc_w(offs_t offset, u8 data);
 	void baud0_w(u8 data);
 	void baud1_w(u8 data);
 
 	void mem_map(address_map &map);
 	void io_map(address_map &map);
+	void video_map(address_map &map);
 
 	required_device<cpu_device> m_maincpu;
 	required_device<fd1793_device> m_fdc;
@@ -55,11 +74,13 @@ private:
 	required_shared_ptr<u8> m_ram;
 
 	bool m_ram_on;
+	bool m_prom_disable;
 };
 
 void gnat10_state::machine_start()
 {
 	save_item(NAME(m_ram_on));
+	save_item(NAME(m_prom_disable));
 }
 
 void gnat10_state::machine_reset()
@@ -69,9 +90,9 @@ void gnat10_state::machine_reset()
 }
 
 
-u8 gnat10_state::ram_r(offs_t offset)
+u8 gnat10_state::mem_r(offs_t offset)
 {
-	if (m_ram_on)
+	if (m_ram_on && (m_prom_disable || offset < 0xf800))
 		return m_ram[offset];
 	else
 		return m_monitor[offset & 0x07ff];
@@ -79,6 +100,10 @@ u8 gnat10_state::ram_r(offs_t offset)
 
 u8 gnat10_state::floppy_status_r()
 {
+	if (!machine().side_effects_disabled())
+	{
+		// TODO: this also forces the motor on (it turns off on a timeout)
+	}
 	return m_fdc->drq_r() << 7 | m_fdc->intrq_r();
 }
 
@@ -93,9 +118,21 @@ void gnat10_state::floppy_latch_w(u8 data)
 	if (floppy != nullptr)
 		floppy->ss_w(BIT(data, 5));
 
+	m_prom_disable = BIT(data, 6);
+
 	m_fdc->mr_w(BIT(data, 7));
 }
 
+u8 gnat10_state::rtc_r(offs_t offset)
+{
+	return 0; //m_rtc->read(offset ^ 0x10);
+}
+
+void gnat10_state::rtc_w(offs_t offset, u8 data)
+{
+	//m_rtc->write(offset ^ 0x10, data);
+}
+
 void gnat10_state::baud0_w(u8 data)
 {
 	m_brg->str_w(data & 0x0f);
@@ -109,19 +146,42 @@ void gnat10_state::baud1_w(u8 data)
 
 void gnat10_state::mem_map(address_map &map)
 {
-	map(0x0000, 0xf7ff).r(FUNC(gnat10_state::ram_r)).writeonly().share("ram");
-	map(0xf800, 0xffff).rom().region("monitor", 0);
+	map(0x0000, 0xffff).r(FUNC(gnat10_state::mem_r)).writeonly().share("ram");
 }
 
 void gnat10_state::io_map(address_map &map)
 {
 	map.global_mask(0xff);
-	map(0x60, 0x63).rw("sio1", FUNC(z80sio_device::ba_cd_r), FUNC(z80sio_device::ba_cd_w));
-	map(0x70, 0x73).rw("sio0", FUNC(z80sio_device::ba_cd_r), FUNC(z80sio_device::ba_cd_w));
-	map(0x80, 0x83).rw(m_fdc, FUNC(fd1793_device::read), FUNC(fd1793_device::write));
-	map(0xa0, 0xa0).rw(FUNC(gnat10_state::floppy_status_r), FUNC(gnat10_state::floppy_latch_w));
-	map(0xd0, 0xd0).w(FUNC(gnat10_state::baud0_w));
-	map(0xe0, 0xe0).w(FUNC(gnat10_state::baud1_w));
+	//map(0x00, 0x0f).rw("dmac", FUNC(am9517a_device::read), FUNC(am9517a_device::write));
+	//map(0x10, 0x11).mirror(0xe).rw("apu", FUNC(am9511_device::read), FUNC(am9511_device::write));
+	//map(0x20, 0x23).mirror(0xc).rw("ctc", FUNC(z80ctc_device::read), FUNC(z80ctc_device::write));
+	//map(0x30, 0x3f).rw("488c", FUNC(tms9914_device::read), FUNC(tms9914_device::write));
+	//map(0x40, 0x40).mirror(0xf).rw(FUNC(gnat10_state::pdma1_r), FUNC(gnat10_state::pdma1_w));
+	//map(0x50, 0x53).mirror(0xc).rw("pio", FUNC(z80pio_device::read), FUNC(z80pio_device::write));
+	map(0x60, 0x63).mirror(0xc).rw("sio1", FUNC(z80sio_device::ba_cd_r), FUNC(z80sio_device::ba_cd_w));
+	map(0x70, 0x73).mirror(0xc).rw("sio0", FUNC(z80sio_device::ba_cd_r), FUNC(z80sio_device::ba_cd_w));
+	map(0x80, 0x83).mirror(0xc).rw(m_fdc, FUNC(fd1793_device::read), FUNC(fd1793_device::write));
+	//map(0x90, 0x90).mirror(0xf).rw(FUNC(gnat10_state::pdma0_r), FUNC(gnat10_state::pdma0_w));
+	map(0xa0, 0xa0).mirror(0xf).rw(FUNC(gnat10_state::floppy_status_r), FUNC(gnat10_state::floppy_latch_w));
+	map(0xb0, 0xcf).rw(FUNC(gnat10_state::rtc_r), FUNC(gnat10_state::rtc_w));
+	map(0xd0, 0xd0).mirror(0xf).w(FUNC(gnat10_state::baud0_w));
+	map(0xe0, 0xe0).mirror(0xf).w(FUNC(gnat10_state::baud1_w));
+	//map(0xf0, 0xf0).r(?); // undocumented switches?
+}
+
+void gnat10_state::video_map(address_map &map)
+{
+	map.global_mask(0x7fff);
+	map(0x0000, 0x07ff).mirror(0x800).rom().region("videocpu", 0);
+	map(0x1000, 0x13ff).mirror(0xc00).ram();
+	map(0x2000, 0x27ff).mirror(0x800).ram().share("vram");
+	//map(0x3000, 0x3000).mirror(0xfff).w(FUNC(gnat10_state::reset_out_w));
+	//map(0x4000, 0x40ff).mirror(0x700).rw("videopio", FUNC(i8155_device::memory_r), FUNC(i8155_device::memory_w));
+	//map(0x4800, 0x4807).mirror(0x7f8).rw("videopio", FUNC(i8155_device::io_r), FUNC(i8155_device::io_w));
+	//map(0x5000, 0x5001).mirror(0xffe).rw("videosio", FUNC(i8251_device::read), FUNC(i8251_device::write));
+	//map(0x6000, 0x6000).mirror(0xfff).w(FUNC(gnat10_state::bell_w));
+	//map(0x7000, 0x7000).mirror(0xffe).w("crtc", FUNC(mc6845_device::address_w));
+	//map(0x7001, 0x7001).mirror(0xffe).rw("crtc", FUNC(mc6845_device::register_r), FUNC(mc6845_device::register_w));
 }
 
 
@@ -146,19 +206,31 @@ void gnat10_state::gnat10(machine_config &config)
 	FLOPPY_CONNECTOR(config, m_floppy[0], gnat_floppies, "525qd", floppy_image_device::default_floppy_formats);
 	FLOPPY_CONNECTOR(config, m_floppy[1], gnat_floppies, "525qd", floppy_image_device::default_floppy_formats);
 
-	COM5016_5(config, m_brg, 4.9152_MHz_XTAL); // specific type unknown
+	COM5016_5(config, m_brg, 4.9152_MHz_XTAL); // BR1941-5
 	m_brg->fr_handler().set("sio0", FUNC(z80sio_device::rxca_w));
 	m_brg->fr_handler().append("sio0", FUNC(z80sio_device::txca_w));
 	m_brg->fr_handler().append("sio1", FUNC(z80sio_device::rxca_w));
 	m_brg->fr_handler().append("sio1", FUNC(z80sio_device::txca_w));
 	m_brg->ft_handler().set("sio1", FUNC(z80sio_device::rxtxcb_w));
 
-	// TODO: video screen, peripheral ports
+	// TODO: DMAC, peripheral ports
+
+	i8085a_cpu_device &videocpu(I8085A(config, "videocpu", 18.432_MHz_XTAL / 4));
+	videocpu.set_addrmap(AS_PROGRAM, &gnat10_state::video_map);
+	videocpu.set_disable();
+
+	// TODO: 8155, 8251A, 6845, video screen
 }
 
 ROM_START(gnat10)
 	ROM_REGION(0x800, "monitor", 0)
 	ROM_LOAD("gnat-507", 0x000, 0x800, CRC(72baa750) SHA1(7b78324b90b8c6f78c88a7dde8d53ea612ea1f7f)) // LF patched back to CR/LF in four instances
+
+	ROM_REGION(0x800, "videocpu", 0)
+	ROM_LOAD("videocpu.bin", 0x000, 0x800, NO_DUMP) // TMS2716 or TMS2732
+
+	ROM_REGION(0x800, "chargen", 0)
+	ROM_LOAD("chargen.bin", 0x000, 0x800, NO_DUMP) // TMS2716 or TMS2732
 ROM_END
 
 COMP(1980, gnat10, 0, 0, gnat10, gnat10, gnat10_state, empty_init, "GNAT Computers", "GNAT System 10", MACHINE_IS_SKELETON)