diff --git a/TODO.md b/TODO.md
deleted file mode 100644
index 988b75c..0000000
--- a/TODO.md
+++ /dev/null
@@ -1,11 +0,0 @@
-- more PIA: check out vice
-
-PIA init-sequence
-5021 > 12 7F
-5021 > 11 A7
-5022 > 13 A7
-5023 > 12 5C
-5025 > 12 D
-5026 > 12 0
-
-http://mamedev.org/source/src/mess/machine/apple1.c.html
diff --git a/io.cpp b/io.cpp
index 16b9a48..30932c2 100644
--- a/io.cpp
+++ b/io.cpp
@@ -12,11 +12,6 @@
 #include "hardware.h"
 #include "config.h"
 
-#define ROWS	24
-#define COLS	40
-static unsigned r, c;
-static char screen[ROWS][COLS];
-
 void io::reset() {
 	Display::begin(BG_COLOUR, FG_COLOUR, ORIENT);
 	clear();
@@ -28,7 +23,7 @@ void io::reset() {
 			screen[j][i] = ' ';
 
 	_loading = false;
-	pia::reset();
+	PIA::reset();
 }
 
 void io::load() {
@@ -78,13 +73,14 @@ static const uint8_t shiftmap[] PROGMEM = {
 };
 
 void io::down(uint8_t scan) {
-	set_porta(0);
 	if (isshift(scan))
 		_shift = true;
 }
 
 void io::enter(uint8_t key) {
-	set_porta(key + 0x80);
+	PIA::write_ca1(false);
+	PIA::write_porta_in(key + 0x80);
+	PIA::write_ca1(true);
 }
 
 void io::up(uint8_t scan) {
@@ -142,25 +138,21 @@ void io::display(uint8_t b) {
 void io::write_portb(uint8_t b) {
 	b &= 0x7f;
 	display(b);
-	pia::write_portb(b);
+	PIA::write_portb(b);
 }
 
-uint8_t io::read_porta_cr() {
-	uint8_t b = pia::read_porta_cr();
-	if (b != 0xa7)
-		return b;
-
+uint8_t io::read_cra() {
 	if (_loading) {
 		if (files.more())
 			enter(files.read());
 		else
 			_loading = false;
 	}
-	return b;
+	return PIA::read_cra();
 }
 
 void io::checkpoint(Stream &s) {
-	pia::checkpoint(s);
+	PIA::checkpoint(s);
 	s.write(r);
 	s.write(c);
 	for (int j = 0; j < ROWS; j++)
@@ -169,7 +161,7 @@ void io::checkpoint(Stream &s) {
 }
 
 void io::restore(Stream &s) {
-	pia::restore(s);
+	PIA::restore(s);
 	r = s.read();
 	c = s.read();
 	for (int j = 0; j < ROWS; j++)
diff --git a/io.h b/io.h
index 9d19df9..619e0ec 100644
--- a/io.h
+++ b/io.h
@@ -1,30 +1,37 @@
 #ifndef _IO_H
 #define _IO_H
 
-// http://mamedev.org/source/src/mess/machine/apple1.c.html
-class io: public Display, Keyboard, public pia {
+class io: public Memory::Device, public Display, Keyboard, public PIA {
 public:
-	io(filer &files): files(files) {}
+	io(filer &files): Memory::Device(Memory::page_size), files(files) {}
 
 	virtual void reset();
-	virtual void down(uint8_t scan);
-	virtual void up(uint8_t scan);
+	virtual void down(uint8_t);
+	virtual void up(uint8_t);
+
+	virtual void operator=(uint8_t b) { PIA::write(_acc, b); }
+	virtual operator uint8_t() { return PIA::read(_acc); }
 
 	virtual void checkpoint(Stream &);
 	virtual void restore(Stream &);
 
 	virtual void write_portb(uint8_t);
-	virtual uint8_t read_porta_cr();
+	virtual uint8_t read_cra();
 
 	void load();
 	filer &files;
+
+	static const uint8_t ROWS = 24;
+	static const uint8_t COLS = 40;
+
 private:
 	void display(uint8_t);
 	void draw(char, int, int);
 	void enter(uint8_t);
 
-	bool _shift;
-	bool _loading;
+	bool _shift, _loading;
+	uint8_t r, c;
+	char screen[ROWS][COLS];
 };
 
 #endif
diff --git a/pia.cpp b/pia.cpp
index a5e06fb..53ba09d 100644
--- a/pia.cpp
+++ b/pia.cpp
@@ -2,60 +2,178 @@
 #include <memory.h>
 #include "pia.h"
 
-void pia::operator=(uint8_t b) {
+// see: https://github.com/mamedev/mame/blob/master/src/devices/machine/6821pia.cpp
+// and: https://github.com/mamedev/mame/blob/master/src/devices/machine/6821pia.h
+
+inline bool c1_low_to_high(uint8_t cr) { return cr & 0x02; }
+
+inline bool c1_high_to_low(uint8_t cr) { return !c1_low_to_high(cr); }
+
+inline bool c2_output(uint8_t cr) { return cr & 0x20; }
+
+inline bool c2_input(uint8_t cr) { return !c2_input(cr); }
+
+inline bool c2_low_to_high(uint8_t cr) { return cr & 0x10; }
+
+inline bool c2_high_to_low(uint8_t cr) { return !c2_low_to_high(cr); }
+
+inline bool output_selected(uint8_t cr) { return cr & 0x04; }
+
+void PIA::write(Memory::address a, uint8_t b) {
 #if defined(DEBUGGING)
 	Serial.print(millis());
 	Serial.print(" > ");
-	Serial.print(_acc, 16);
+	Serial.print(a, 16);
 	Serial.print(' ');
 	Serial.println(b, 16);
 #endif
-	switch(_acc % 4) {
+	switch(a % 4) {
 	case 0:
-		write_porta(b);
+		output_selected(cra)? write_porta(b): write_ddra(b);
 		break;
 	case 1:
-		write_porta_cr(b);
+		write_cra(b);
 		break;
 	case 2:
-		write_portb(b);
+		output_selected(crb)? write_portb(b): write_ddrb(b);
 		break;
 	case 3:
-		write_portb_cr(b);
+		write_crb(b);
 		break;
 	}
 }
 
-pia::operator uint8_t() {
+uint8_t PIA::read(Memory::address a) {
 #if defined(DEBUGGING)
 	Serial.print(millis());
 	Serial.print(" < ");
-	Serial.println(_acc, 16);
+	Serial.println(a, 16);
 #endif
-	switch (_acc % 4) {
+	switch (a % 4) {
 	case 0:
-		return read_porta();
+		return output_selected(cra)? read_porta(): read_ddra();
 	case 1:
-		return read_porta_cr();
+		return read_cra();
 	case 2:
-		return read_portb();
+		return output_selected(crb)? read_portb(): read_ddrb();
 	case 3:
-		return read_portb_cr();
+		return read_crb();
 	}
 	return 0xff;
 }
 
-void pia::checkpoint(Stream &s) {
-	s.write(portb_cr);
-	s.write(portb);
-	s.write(porta_cr);
-	s.write(porta);
+void PIA::checkpoint(Stream &s) {
+	s.write(crb);
+	s.write(outb);
+	s.write(inb);
+	s.write(ddrb);
+	s.write(cra);
+	s.write(outa);
+	s.write(ina);
+	s.write(ddra);
+	s.write(irq_b1);
+	s.write(irq_b2);
+	s.write(irq_a1);
+	s.write(irq_a2);
+	s.write(cb1);
+	s.write(cb2);
+	s.write(ca1);
+	s.write(ca2);
 }
 
-void pia::restore(Stream &s) {
-	portb_cr = s.read();
-	portb = s.read();
-	porta_cr = s.read();
-	porta = s.read();
+void PIA::restore(Stream &s) {
+	crb = s.read();
+	outb = s.read();
+	inb = s.read();
+	ddrb = s.read();
+	cra = s.read();
+	outa = s.read();
+	ina = s.read();
+	ddra = s.read();
+	irq_b1 = s.read();
+	irq_b2 = s.read();
+	irq_a1 = s.read();
+	irq_a2 = s.read();
+	cb1 = s.read();
+	cb2 = s.read();
+	ca1 = s.read();
+	ca2 = s.read();
 }
 
+void PIA::write_ca1(bool state) {
+
+	if (ca1 == state)
+		return;
+
+	if ((state && c1_low_to_high(cra)) || (!state && c1_high_to_low(cra)))
+		irq_a1 = true;
+
+	ca1 = state;
+}
+
+void PIA::write_ca2(bool state) {
+
+	if (ca2 == state || !c2_input(cra))
+		return;
+
+	if ((state && c2_low_to_high(cra)) || (!state && c2_high_to_low(cra)))
+		irq_a2 = true;
+
+	ca2 = state;
+}
+
+void PIA::write_cb1(bool state) {
+
+	if (cb1 == state)
+		return;
+
+	if ((state && c1_low_to_high(crb)) || (!state && c1_high_to_low(crb)))
+		irq_b1 = true;
+
+	cb1 = state;
+}
+
+void PIA::write_cb2(bool state) {
+
+	if (cb2 == state || !c2_input(crb))
+		return;
+
+	if ((state && c2_low_to_high(crb)) || (!state && c2_high_to_low(crb)))
+		irq_b2 = true;
+
+	cb2 = state;
+}
+
+uint8_t PIA::read_cra() {
+	uint8_t b = cra;
+
+	if (irq_a1)
+		b |= IRQ1;
+
+	if (irq_a2 && c2_input(cra))
+		b |= IRQ2;
+
+	return b;
+}
+
+uint8_t PIA::read_crb() {
+	uint8_t b = crb;
+
+	if (irq_b1)
+		b |= IRQ1;
+
+	if (irq_b2 && c2_input(crb))
+		b |= IRQ2;
+
+	return b;
+}
+
+uint8_t PIA::read_porta() {
+	irq_a1 = irq_a2 = false;
+	return (ina & ~ddra) | (outa & ddra);
+}
+
+uint8_t PIA::read_portb() {
+	irq_b1 = irq_b2 = false;
+	return (inb & ~ddrb) | (outb & ddrb);
+}
diff --git a/pia.h b/pia.h
index 1818429..8104ff1 100644
--- a/pia.h
+++ b/pia.h
@@ -1,51 +1,57 @@
 #ifndef __PIA_H__
 #define __PIA_H__
 
-class pia: public Memory::Device {
+// Motorola 6820 / 6821 PIA
+// https://en.wikipedia.org/wiki/Peripheral_Interface_Adapter
+class PIA {
 public:
-	pia(): Memory::Device(256), portb_cr(0), porta_cr(0) {}
-	virtual void reset() { portb_cr = porta_cr = 0; }
+	PIA(): outb(0), inb(0), crb(0), outa(0), ina(0), cra(0), ddrb(0), ddra(0),
+		ca1(false), ca2(false), cb1(false), cb2(false),
+		irq_a1(false), irq_a2(false), irq_b1(false), irq_b2(false) {}
 
-	void operator=(uint8_t);
-	operator uint8_t();
+	virtual void reset() {
+		outb = inb = crb = ddrb = outa = ina = cra = ddra = 0;
+		irq_a1 = irq_a2 = irq_b1 = irq_b2 = ca1 = ca2 = cb1 = cb2 = false;
+	}
+
+	// device memory interface
+	void write(Memory::address, uint8_t);
+	uint8_t read(Memory::address);
 
 	void checkpoint(Stream &);
 	void restore(Stream &);
 
+	// device input (external) interface
+	void write_porta_in(uint8_t b) { ina = b; }
+	void write_ca1(bool);
+	void write_ca2(bool);
+	void write_portb_in(uint8_t b) { inb = b; }
+	void write_cb1(bool);
+	void write_cb2(bool);
+
+	static const uint8_t IRQ1 = 0x80;
+	static const uint8_t IRQ2 = 0x40;
+
 protected:
-	// write to the "external" side of the port
-	void set_porta(uint8_t b) {
-		porta = b;
-		if (b & 0x80)
-			porta_cr = 0xa7;
-	}
+	// overrideable device memory interface
+	virtual uint8_t read_ddra() { return ddra; }
+	virtual uint8_t read_porta();
+	virtual uint8_t read_cra();
+	virtual uint8_t read_ddrb() { return ddrb; }
+	virtual uint8_t read_portb();
+	virtual uint8_t read_crb();
 
-	// "device-side" operations (called from memory interface)
-	uint8_t read_porta() { return porta; }
-	virtual uint8_t read_porta_cr() {
-		if (porta_cr & 0x80) {
-			porta_cr = 0;
-			return 0xa7;
-		}
-		return porta_cr;
-	}
-	uint8_t read_portb() { return portb; }
-	uint8_t read_portb_cr() { return portb_cr; }
-
-	void write_porta(uint8_t b) { porta = b; }
-	void write_porta_cr(uint8_t b) {
-		if (!(porta_cr & 0x80) && b >= 0x80)
-			porta_cr |= 0x80;
-		else
-			porta_cr = b;
-	}
-	virtual void write_portb(uint8_t b) { portb = b; }
-	void write_portb_cr(uint8_t b) {
-		if (portb_cr < 0x80)
-			portb_cr = b;
-	}
+	virtual void write_ddra(uint8_t b) { ddra = b; }
+	virtual void write_porta(uint8_t b) { outa = b; }
+	virtual void write_cra(uint8_t b) { cra = (b & 0x3f); }
+	virtual void write_ddrb(uint8_t b) { ddrb = b; }
+	virtual void write_portb(uint8_t b) { outb = b; }
+	virtual void write_crb(uint8_t b) { crb = (b & 0x3f); }
 
 private:
-	uint8_t portb_cr, portb, porta_cr, porta;
+	uint8_t cra, ina, outa, ddra;
+	uint8_t crb, inb, outb, ddrb;
+	bool ca1, ca2, irq_a1, irq_a2;
+	bool cb1, cb2, irq_b1, irq_b2;
 };
 #endif