diff --git a/vimlac/Makefile b/vimlac/Makefile
index ecdad67..4bb3c53 100644
--- a/vimlac/Makefile
+++ b/vimlac/Makefile
@@ -1,4 +1,6 @@
-OFILES = vimlac.o cpu.o dcpu.o ptr.o ptp.o memory.o kb.o ttyin.o ttyout.o trace.o
+OFILES=cpu.o dcpu.o ptr.o ptp.o memory.o kbd.o ttyin.o ttyout.o trace.o
+CFILES=cpu.c dcpu.c ptr.c ptp.c memory.c kbd.c ttyin.c ttyout.c trace.c error.c
+HFILES=cpu.h dcpu.h ptr.h ptp.h memory.h kbd.h ttyin.h ttyout.h trace.h error.h
 
 CFLAGS=-shared -fPIC -O2 -Wall -ansi -pedantic -std=c99 -g
 
@@ -10,6 +12,9 @@ vimlac:	${OFILES} Makefile
 	#gcc -o vimlac.so -shared -fPIC ${OFILES}
 	gcc -o vimlac ${OFILES}
 
+test_cpu:	test_cpu.c $(CFILES) $(HFILES) Makefile
+	gcc -o test_cpu test_cpu.c $(CFILES)
+
 test_ptr:	test_ptr.c ptr.c ptr.h Makefile
 	gcc -o test_ptr ptr.c test_ptr.c
 
diff --git a/vimlac/cpu.c b/vimlac/cpu.c
index d0fd949..eae29d8 100755
--- a/vimlac/cpu.c
+++ b/vimlac/cpu.c
@@ -6,11 +6,11 @@
  *                                                                            *
 \******************************************************************************/
 
-#include "vimlac.h"
+#include "imlac.h"
 #include "cpu.h"
 #include "dcpu.h"
 #include "memory.h"
-#include "kb.h"
+#include "kbd.h"
 #include "ptr.h"
 #include "ptp.h"
 #include "ttyin.h"
@@ -492,7 +492,7 @@ Description : Emulate the DSF instruction.
 static int
 i_DSF(void)
 {
-    if (dcpu_on())
+    if (dcpu_running())
         r_PC = (r_PC + 1) & WORD_MASK;
 
     trace("DSF\t");
@@ -529,7 +529,7 @@ Description : Emulate the DSN instruction.
 static int
 i_DSN(void)
 {
-    if (!dcpu_on())
+    if (!dcpu_running())
         r_PC = (r_PC + 1) & WORD_MASK;
 
     trace("DSN\t");
@@ -585,7 +585,7 @@ Description : Emulate the IMLAC KCF instruction.
 static int
 i_KCF(void)
 {
-    kb_clear_flag();
+    kbd_clear_flag();
 
     trace("KCF\t");
 
@@ -602,7 +602,7 @@ Description : Emulate the IMLAC KRB instruction.
 static int
 i_KRB(void)
 {
-    r_AC |= kb_get_char();
+    r_AC |= kbd_get_char();
 
     trace("KRB\t");
 
@@ -619,8 +619,8 @@ Description : Emulate the IMLAC KRC instruction.
 static int
 i_KRC(void)
 {
-    r_AC |= kb_get_char();
-    kb_clear_flag();
+    r_AC |= kbd_get_char();
+    kbd_clear_flag();
 
     trace("KRC\t");
 
@@ -637,7 +637,7 @@ Description : Emulate the IMLAC KSF instruction.
 static int
 i_KSF(void)
 {
-    if (kb_ready())
+    if (kbd_ready())
         r_PC = (r_PC + 1) & WORD_MASK;
 
     trace("KSF\t");
@@ -655,7 +655,7 @@ Description : Emulate the IMLAC KSN instruction.
 static int
 i_KSN(void)
 {
-    if (!kb_ready())
+    if (!kbd_ready())
         r_PC = (r_PC + 1) & WORD_MASK;
 
     trace("KSN\t");
@@ -673,7 +673,9 @@ Description : Emulate the IMLAC PUN instruction.
 static int
 i_PUN(void)
 {
-    ptp_punch(r_AC & 0xff);
+    char value = r_AC & 0xff;
+
+    ptp_punch(value);
 
     trace("PUN\t");
 
@@ -937,7 +939,9 @@ Description : Emulate the IMLAC TPC instruction.
 static int
 i_TPC(void)
 {
-    ttyout_send(r_AC & 0xff);
+    BYTE value = r_AC & 0xff;
+
+    ttyout_send(value);
     ttyout_clear_flag();
 
     trace("TPC\t");
@@ -1116,7 +1120,7 @@ Description : Emulate the IMLAC DLA instruction.
 static int
 i_DLA(void)
 {
-    dcpu_set_PC(r_AC);
+    dcpu_set_DPC(r_AC);
 
     trace("DLA\t");
 
diff --git a/vimlac/cpu.h b/vimlac/cpu.h
index 93b34b2..e532e06 100755
--- a/vimlac/cpu.h
+++ b/vimlac/cpu.h
@@ -1,12 +1,15 @@
 /******************************************************************************\
  *                                  cpu.h                                     *
  *                                 -------                                    *
+ *                                                                            *
+ *  Implements all main CPU instructions.                                     *
+ *                                                                            *
 \******************************************************************************/
 
 #ifndef CPU_H
 #define CPU_H
 
-#include "vimlac.h"
+#include "imlac.h"
 
 /******
  * Exported functions.
diff --git a/vimlac/dcpu.c b/vimlac/dcpu.c
new file mode 100755
index 0000000..9ead4ed
--- /dev/null
+++ b/vimlac/dcpu.c
@@ -0,0 +1,181 @@
+/******************************************************************************\
+ *                                 dcpu.c                                     *
+ *                                --------                                    *
+ *                                                                            *
+ *  This file is used to decode and execute a display processor instruction.  *
+ *                                                                            *
+\******************************************************************************/
+
+#include "imlac.h"
+#include "dcpu.h"
+#include "memory.h"
+#include "trace.h"
+
+
+/******
+ * Emulated registers, state, memory, etc.
+ ******/
+
+static WORD     r_DPC;
+static WORD     Prev_r_DPC;
+static WORD     DRSindex = 0;
+
+/* 40Hz sync stuff */
+static bool            Sync40HzOn = false;
+
+/******
+ * Environment stuff.  PTR and TTY in and out files, etc
+ ******/
+
+static bool            dcpu_on;       /* true if display processor is running */
+static bool            dcpu_sync_on;  /* true if 40HZ flag set */
+
+
+/******************************************************************************
+Description : Function to get the display CPU status.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+bool
+dcpu_running(void)
+{
+    return dcpu_on == true;
+}
+
+
+/******************************************************************************
+Description : Function to start the display CPU.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void
+dcpu_start(void)
+{
+    dcpu_on = true;
+}
+
+
+/******************************************************************************
+Description : Function to stop the display CPU.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void
+dcpu_stop(void)
+{
+    dcpu_on = false;
+}
+
+
+/******************************************************************************
+Description : Functions to get various registers.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+WORD
+dcpu_get_DPC(void)
+{
+    return r_DPC;
+}
+
+
+WORD
+dcpu_get_prev_DPC(void)
+{
+    return Prev_r_DPC;
+}
+
+
+void
+dcpu_set_DPC(WORD new_dpc)
+{
+    r_DPC = new_dpc;
+}
+
+void
+dcpu_set_DRSindex(WORD drsindex)
+{
+    DRSindex = drsindex;
+}
+
+
+/******************************************************************************
+Description : Function to handle unrecognized instruction.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+static void
+illegal(void)
+{
+    WORD oldDPC = Prev_r_DPC & MEMMASK;
+
+    error("INTERNAL ERROR: "
+          "unexpected display processor opcode %06.6o at address %06.6o",
+          mem_get(oldDPC, false), oldDPC);
+}
+
+
+/******************************************************************************
+Description : 
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+static int
+i_LAW_LWC(bool indirect, WORD address)
+{
+    return 1;
+}
+
+
+/******************************************************************************
+Description : Function to execute one display processor instruction.
+ Parameters : 
+    Returns : The number of cycles the instruction took.
+   Comments : 
+ ******************************************************************************/
+int
+dcpu_execute_one(void)
+{
+    WORD instruction;
+    WORD opcode;
+    WORD address;
+    bool indirect;
+
+/******
+ * If main processor not running, return immediately.
+ ******/
+
+    if (!dcpu_on)
+        return 0;
+
+/******
+ * Fetch the instruction.  Split into initial opcode and address.
+ ******/
+
+    Prev_r_DPC = r_DPC;
+    instruction = mem_get(r_DPC++, false);
+    r_DPC = r_DPC & MEMMASK;
+
+    indirect = (bool) (instruction & 0100000);	/* high bit set? */
+    opcode = (instruction >> 11) & 017;		/* high 5 bits */
+    address = instruction & 03777;		/* low 11 bits */
+
+/******
+ * Now decode it.
+ ******/
+
+    switch (opcode)
+    {
+	default: illegal();
+    }
+
+    return 0;	/* CAN'T REACH */
+}
+
+
diff --git a/vimlac/dcpu.h b/vimlac/dcpu.h
new file mode 100755
index 0000000..d602f2d
--- /dev/null
+++ b/vimlac/dcpu.h
@@ -0,0 +1,28 @@
+/******************************************************************************\
+ *                                  dcpu.h                                    *
+ *                                 --------                                   *
+ *                                                                            *
+ *  Implements all display CPU instructions.                                  *
+ *                                                                            *
+\******************************************************************************/
+
+#ifndef DCPU_H
+#define DCPU_H
+
+#include "imlac.h"
+
+/******
+ * Exported functions.
+ ******/
+
+bool dcpu_running(void);
+void dcpu_start(void);
+void dcpu_stop(void);
+void dcpu_set_DRSindex(WORD);
+int dcpu_execute_one(void);
+WORD dcpu_get_AC(void);
+WORD dcpu_get_DPC(void);
+WORD dcpu_get_prev_DPC(void);
+void dcpu_set_DPC(WORD dpc);
+
+#endif
diff --git a/vimlac/error.c b/vimlac/error.c
new file mode 100644
index 0000000..d3b0bc8
--- /dev/null
+++ b/vimlac/error.c
@@ -0,0 +1,30 @@
+/*
+ * Error routines for the imlac simulator.
+ */
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+
+/******************************************************************************
+Description : printf()-style trace routine.
+ Parameters : like printf()
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void
+error(char *fmt, ...)
+{
+    va_list ap;
+    char    buff[1024];
+
+    va_start(ap, fmt);
+    vsprintf(buff, fmt, ap);
+    fprintf(stderr, "%s\n", buff);
+    va_end(ap);
+
+    exit(10);
+}
+
+
diff --git a/vimlac/error.h b/vimlac/error.h
new file mode 100644
index 0000000..36720ed
--- /dev/null
+++ b/vimlac/error.h
@@ -0,0 +1,10 @@
+/*
+ * Error routines for the imlac simulation.
+ */
+
+#ifndef ERROR_H
+#define ERROR_H
+
+void error(char *fmt, ...);
+
+#endif
diff --git a/vimlac/kbd.c b/vimlac/kbd.c
new file mode 100755
index 0000000..a9f0304
--- /dev/null
+++ b/vimlac/kbd.c
@@ -0,0 +1,58 @@
+/******************************************************************************\
+ *                                 kbd.c                                      *
+ *                                -------                                     *
+ *                                                                            *
+ *  This file is used to implement a KBD device (keyboard).                   *
+ *                                                                            *
+\******************************************************************************/
+
+#include "imlac.h"
+#include "kbd.h"
+#include "trace.h"
+
+
+/******
+ * Emulated registers, state, memory, etc.
+ ******/
+
+static WORD     state_KBD;
+static bool     clear = false;
+
+
+/******************************************************************************
+Description : Clear the KBD (keyboard) flag.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void
+kbd_clear_flag(void)
+{
+    clear = false;
+}
+
+/******************************************************************************
+Description : Get the character in the keyboard buffer.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+WORD
+kbd_get_char(void)
+{
+    return 'a';
+}
+
+/******************************************************************************
+Description : Test if the keyboard is ready with the next character.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+bool
+kbd_ready(void)
+{
+    return !clear;
+}
+
+
diff --git a/vimlac/kbd.h b/vimlac/kbd.h
new file mode 100755
index 0000000..fa87a3a
--- /dev/null
+++ b/vimlac/kbd.h
@@ -0,0 +1,24 @@
+/******************************************************************************\
+ *                                  kbd.h                                     *
+ *                                 -------                                    *
+ *                                                                            *
+ *  Implements all display CPU instructions.                                  *
+ *                                                                            *
+\******************************************************************************/
+
+#ifndef KBD_H
+#define KBD_H
+
+#include "imlac.h"
+
+/******
+ * Exported functions.
+ ******/
+
+
+void kbd_clear_flag(void);
+WORD kbd_get_char(void);
+bool kbd_ready(void);
+
+
+#endif
diff --git a/vimlac/ptp.c b/vimlac/ptp.c
new file mode 100644
index 0000000..503ce76
--- /dev/null
+++ b/vimlac/ptp.c
@@ -0,0 +1,159 @@
+/*
+ * Implementation for the imlac PTP (papertape punch) device.
+ */
+
+#include "imlac.h"
+#include "ptp.h"
+
+
+/*****
+ * constants for the PTP device
+ *
+ * The device punches at 100 chars/second, so we work out how many imlac
+ * machine cycles data is ready/notready at a 30%/70% ready cycle.
+ ******/
+
+#define CHARS_PER_SECOND    100
+#define CYCLES_PER_CHAR     (CPU_HERZ / CHARS_PER_SECOND)
+#define READY_CYCLES        (int)((3 * CYCLES_PER_CHAR) / 10)
+#define NOT_READY_CYCLES    (int)((7 * CYCLES_PER_CHAR) / 10)
+
+/*****
+ * State variables for the PTP device
+ ******/
+
+static bool motor_on = false;
+static bool device_ready = false;
+static FILE *open_file;
+static char *filename = NULL;
+static long cycle_count = 0;
+static char value = 0;
+
+
+
+/******************************************************************************
+Description : Mount a papertape file on the PTP device
+ Parameters : fname - pathname of the file to mount
+    Returns : 0 if no error, else status code.
+   Comments : 
+ ******************************************************************************/
+int ptp_mount(char *fname)
+{
+    filename = fname;
+    open_file = fopen(fname, "wb");
+    if (open_file == NULL)
+    {
+        ptp_dismount();
+        return errno;
+    }
+    motor_on = false;
+    device_ready = false;
+    cycle_count = NOT_READY_CYCLES;
+
+    return 0;
+}
+
+
+/******************************************************************************
+Description : Dismount papertape file from device.
+ Parameters : 
+    Returns : 
+   Comments : Turns motor off.
+ ******************************************************************************/
+void ptp_dismount(void)
+{
+    if (open_file)
+        if (fclose(open_file) != 0)
+    filename = NULL;
+    open_file = NULL;
+    motor_on = false;
+    device_ready = false;
+}
+
+
+/******************************************************************************
+Description : Start the papertape device motor.
+ Parameters : 
+    Returns : 
+   Comments : We don't check if papertape mounted as the real imlac doesn't.
+ ******************************************************************************/
+void ptp_start(void)
+{
+    motor_on = true;
+    device_ready = false;
+    cycle_count = NOT_READY_CYCLES;
+}
+
+
+/******************************************************************************
+Description : Turn the papertape device motor off.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void ptp_stop(void)
+{
+    motor_on = false;
+    device_ready = false;
+    cycle_count = NOT_READY_CYCLES;
+}
+
+
+/******************************************************************************
+Description : Write the current value to the papertape punch file.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+int ptp_punch(BYTE value)
+{
+    return value;
+}
+
+
+/******************************************************************************
+Description : Get the papertape device status.
+ Parameters : 
+    Returns : TRUE if device ready, else FALSE.
+   Comments : 
+ ******************************************************************************/
+bool ptp_ready(void)
+{
+    return device_ready;
+}
+
+
+/******************************************************************************
+Description : Tick the state machine along a bit.
+ Parameters : cycles - number of imlac cycles that have elapsed
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void ptp_tick(long cycles)
+{
+    /* if no state change */
+    if (!motor_on || open_file == NULL)
+        return;
+
+    /* tape in, motor on */
+    cycle_count -= cycles;
+    if (cycle_count <= 0L)
+    {
+        if (device_ready)
+        {
+            device_ready = false;
+            cycle_count += NOT_READY_CYCLES;
+	    value = 0;
+        }
+        else
+        {
+            device_ready = true;
+            cycle_count += READY_CYCLES;
+            if (fread(&value, sizeof(BYTE), 1, open_file) != 1)
+            {   /* assume EOF on file, dismount tape */
+		fclose(open_file);
+		open_file = NULL;
+            }
+        }
+    }
+}
diff --git a/vimlac/ptp.h b/vimlac/ptp.h
new file mode 100644
index 0000000..3c3c944
--- /dev/null
+++ b/vimlac/ptp.h
@@ -0,0 +1,16 @@
+/*
+ * Interface for the imlac PTP (papertape punch).
+ */
+
+#ifndef PTP_H
+#define PTP_H
+
+int ptp_mount(char *fname);
+void ptp_dismount(void);
+void ptp_start(void);
+void ptp_stop(void);
+int ptp_punch(BYTE value);
+bool ptp_ready(void);
+void ptp_tick(long cycles);
+
+#endif
diff --git a/vimlac/ptr.c b/vimlac/ptr.c
index 41fb647..a7ce533 100644
--- a/vimlac/ptr.c
+++ b/vimlac/ptr.c
@@ -1,15 +1,15 @@
 /*
- * Implementation for the vimlac PTR (papertape reader).
+ * Implementation for the imlac PTR (papertape reader) device.
  */
 
-#include "vimlac.h"
+#include "imlac.h"
 #include "ptr.h"
 
 
 /*****
  * constants for the PTR device
  *
- * The device reads at 300 chars/second, so we work out how many
+ * The device reads at 300 chars/second, so we work out how many imlac
  * machine cycles data is ready/notready at a 30%/70% ready cycle.
  ******/
 
@@ -33,6 +33,13 @@ static BYTE value = PTR_EOF;
 static long cycle_count = 0;
 
 
+
+/******************************************************************************
+Description : Mount a papertape file on the PTR device
+ Parameters : fname - pathname of the file to mount
+    Returns : 0 if no error, else status code.
+   Comments : 
+ ******************************************************************************/
 int ptr_mount(char *fname)
 {
     filename = fname;
@@ -52,6 +59,12 @@ int ptr_mount(char *fname)
 }
 
 
+/******************************************************************************
+Description : Dismount papertape file from device.
+ Parameters : 
+    Returns : 
+   Comments : Turns motor off.
+ ******************************************************************************/
 void ptr_dismount(void)
 {
     if (open_file)
@@ -65,6 +78,12 @@ void ptr_dismount(void)
 }
 
 
+/******************************************************************************
+Description : Start the papertape device motor.
+ Parameters : 
+    Returns : 
+   Comments : We don't check if papertape mounted as the real imlac doesn't.
+ ******************************************************************************/
 void ptr_start(void)
 {
     motor_on = true;
@@ -73,6 +92,12 @@ void ptr_start(void)
 }
 
 
+/******************************************************************************
+Description : Turn the papertape device motor off.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
 void ptr_stop(void)
 {
     motor_on = false;
@@ -80,18 +105,36 @@ void ptr_stop(void)
 }
 
 
+/******************************************************************************
+Description : Read the current value of the papertape device.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
 int ptr_read(void)
 {
     return value;
 }
 
 
+/******************************************************************************
+Description : Get the papertape device status.
+ Parameters : 
+    Returns : TRUE if device ready, else FALSE.
+   Comments : 
+ ******************************************************************************/
 bool ptr_ready(void)
 {
     return device_ready;
 }
 
 
+/******************************************************************************
+Description : Tick the state machine along a bit.
+ Parameters : cycles - number of imlac cycles that have elapsed
+    Returns : 
+   Comments : 
+ ******************************************************************************/
 void ptr_tick(long cycles)
 {
     /* if no state change */
diff --git a/vimlac/ptr.h b/vimlac/ptr.h
index d555b8b..d3612a9 100644
--- a/vimlac/ptr.h
+++ b/vimlac/ptr.h
@@ -1,5 +1,5 @@
 /*
- * Interface for the vimlac PTR (papertape reader).
+ * Interface for the imlac PTR (papertape reader).
  */
 
 #ifndef PTR_H
diff --git a/vimlac/test_cpu.c b/vimlac/test_cpu.c
new file mode 100644
index 0000000..383acf9
--- /dev/null
+++ b/vimlac/test_cpu.c
@@ -0,0 +1,23 @@
+/*
+ * Test the CPU implementation.
+ */
+
+#include <stdio.h>
+#include <assert.h>
+
+#include "imlac.h"
+#include "cpu.h"
+#include "memory.h"
+
+#define trace(args) ;
+
+
+int
+main(int argc, char *argv[])
+{
+    int errors = 0;
+
+    printf("%d errors found\n", errors);
+
+    return 0;
+}
diff --git a/vimlac/trace.c b/vimlac/trace.c
new file mode 100644
index 0000000..c5dd175
--- /dev/null
+++ b/vimlac/trace.c
@@ -0,0 +1,27 @@
+/*
+ * Trace routines for the imlac simulator.
+ */
+
+#include <stdarg.h>
+#include <stdio.h>
+
+
+/******************************************************************************
+Description : printf()-style trace routine.
+ Parameters : like printf()
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void
+trace(char *fmt, ...)
+{
+    va_list ap;
+    char    buff[1024];
+
+    va_start(ap, fmt);
+    vsprintf(buff, fmt, ap);
+    fprintf(stdout, "%s\n", buff);
+    va_end(ap);
+}
+
+
diff --git a/vimlac/trace.h b/vimlac/trace.h
new file mode 100644
index 0000000..646623d
--- /dev/null
+++ b/vimlac/trace.h
@@ -0,0 +1,10 @@
+/*
+ * Interface for the imlac TRACE routine(s).
+ */
+
+#ifndef TRACE_H
+#define TRACE_H
+
+void trace(char *fmt, ...);
+
+#endif
diff --git a/vimlac/ttyin.c b/vimlac/ttyin.c
new file mode 100644
index 0000000..0b8ebc2
--- /dev/null
+++ b/vimlac/ttyin.c
@@ -0,0 +1,149 @@
+/*
+ * Implementation for the imlac TTYIN (TTY input) device.
+ */
+
+#include "imlac.h"
+#include "ttyin.h"
+
+
+/*****
+ * constants for the TTYIN device
+ *
+ * The device reads at 4800 chars/second, so we work out how many imlac
+ * machine cycles data is ready/notready at a 30%/70% ready cycle.
+ ******/
+
+#define CHARS_PER_SECOND    4800
+#define CYCLES_PER_CHAR     (CPU_HERZ / CHARS_PER_SECOND)
+#define READY_CYCLES        (int)((3 * CYCLES_PER_CHAR) / 10)
+#define NOT_READY_CYCLES    (int)((7 * CYCLES_PER_CHAR) / 10)
+
+#define TTYIN_EOF           0377
+
+/*****
+ * State variables for the TTYIN device
+ ******/
+
+static bool device_ready = false;
+static FILE *open_file;
+static char *filename = NULL;
+static bool at_eof = false;
+static BYTE value = TTYIN_EOF;
+static long cycle_count = 0;
+
+
+
+/******************************************************************************
+Description : Mount a papertape file on the TTYIN device
+ Parameters : fname - pathname of the file to mount
+    Returns : 0 if no error, else status code.
+   Comments : 
+ ******************************************************************************/
+int ttyin_mount(char *fname)
+{
+    filename = fname;
+    open_file = fopen(fname, "rb");
+    if (open_file == NULL)
+    {
+        ttyin_dismount();
+        return errno;
+    }
+    device_ready = false;
+    at_eof = false;
+    value = TTYIN_EOF;
+    cycle_count = NOT_READY_CYCLES;
+
+    return 0;
+}
+
+
+/******************************************************************************
+Description : Dismount papertape file from device.
+ Parameters : 
+    Returns : 
+   Comments : Turns motor off.
+ ******************************************************************************/
+void ttyin_dismount(void)
+{
+    if (open_file)
+        if (fclose(open_file) != 0)
+    filename = NULL;
+    open_file = NULL;
+    device_ready = true;
+    at_eof = true;
+    value = TTYIN_EOF;
+}
+
+
+/******************************************************************************
+Description : Read the current value of the papertape device.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+int ttyin_get_char(void)
+{
+    return value;
+}
+
+
+/******************************************************************************
+Description : Get the papertape device status.
+ Parameters : 
+    Returns : TRUE if device ready, else FALSE.
+   Comments : 
+ ******************************************************************************/
+void ttyin_clear_flag(void)
+{
+    device_ready = false;
+}
+
+
+/******************************************************************************
+Description : Get the papertape device status.
+ Parameters : 
+    Returns : TRUE if device ready, else FALSE.
+   Comments : 
+ ******************************************************************************/
+bool ttyin_ready(void)
+{
+    return device_ready;
+}
+
+
+/******************************************************************************
+Description : Tick the state machine along a bit.
+ Parameters : cycles - number of imlac cycles that have elapsed
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void ttyin_tick(long cycles)
+{
+    /* if no state change */
+    if (at_eof || open_file == NULL)
+        return;
+
+    /* tape in, motor on */
+    cycle_count -= cycles;
+    if (cycle_count <= 0L)
+    {
+        if (device_ready)
+        {
+            device_ready = false;
+            cycle_count += NOT_READY_CYCLES;
+	    value = 0;
+        }
+        else
+        {
+            device_ready = true;
+            cycle_count += READY_CYCLES;
+            if (fread(&value, sizeof(BYTE), 1, open_file) != 1)
+            {   /* assume EOF on file, dismount tape */
+		fclose(open_file);
+		open_file = NULL;
+                at_eof = true;
+                value = TTYIN_EOF;
+            }
+        }
+    }
+}
diff --git a/vimlac/ttyin.h b/vimlac/ttyin.h
new file mode 100644
index 0000000..c4597a9
--- /dev/null
+++ b/vimlac/ttyin.h
@@ -0,0 +1,15 @@
+/*
+ * Interface for the imlac TTYIN (TTY input).
+ */
+
+#ifndef TTYIN_H
+#define TTYIN_H
+
+int ttyin_mount(char *fname);
+void ttyin_dismount(void);
+int ttyin_get_char(void);
+void ttyin_tick(long cycles);
+bool ttyin_ready(void);
+void ttyin_clear_flag(void);
+
+#endif
diff --git a/vimlac/ttyout.c b/vimlac/ttyout.c
new file mode 100644
index 0000000..a0f6dd7
--- /dev/null
+++ b/vimlac/ttyout.c
@@ -0,0 +1,136 @@
+/*
+ * Implementation for the imlac TTYOUT (TTY output) device.
+ */
+
+#include "imlac.h"
+#include "ttyout.h"
+
+
+/*****
+ * constants for the TTYOUT device
+ *
+ * The device reads at 4800 chars/second, so we work out how many imlac
+ * machine cycles data is ready/notready at a 30%/70% ready cycle.
+ ******/
+
+#define CHARS_PER_SECOND    4800
+#define CYCLES_PER_CHAR     (CPU_HERZ / CHARS_PER_SECOND)
+#define READY_CYCLES        (int)((3 * CYCLES_PER_CHAR) / 10)
+#define NOT_READY_CYCLES    (int)((7 * CYCLES_PER_CHAR) / 10)
+
+#define TTYOUT_EOF           0377
+
+/*****
+ * State variables for the TTYOUT device
+ ******/
+
+static bool device_ready = false;
+static FILE *open_file;
+static char *filename = NULL;
+static long cycle_count = 0;
+
+
+
+/******************************************************************************
+Description : Mount a papertape file on the TTYOUT device
+ Parameters : fname - pathname of the file to mount
+    Returns : 0 if no error, else status code.
+   Comments : 
+ ******************************************************************************/
+int ttyout_mount(char *fname)
+{
+    filename = fname;
+    open_file = fopen(fname, "wb");
+    if (open_file == NULL)
+    {
+        ttyout_dismount();
+        return errno;
+    }
+    device_ready = false;
+    cycle_count = NOT_READY_CYCLES;
+
+    return 0;
+}
+
+
+/******************************************************************************
+Description : Dismount papertape file from device.
+ Parameters : 
+    Returns : 
+   Comments : Turns motor off.
+ ******************************************************************************/
+void ttyout_dismount(void)
+{
+    if (open_file)
+        if (fclose(open_file) != 0)
+    filename = NULL;
+    open_file = NULL;
+    device_ready = true;
+}
+
+
+/******************************************************************************
+Description : Write a byte value to the device.
+ Parameters : 
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void ttyout_send(BYTE value)
+{
+    if (device_ready)
+        fwrite(&value, 1, 1, open_file);
+}
+
+
+/******************************************************************************
+Description : Get the papertape device status.
+ Parameters : 
+    Returns : TRUE if device ready, else FALSE.
+   Comments : 
+ ******************************************************************************/
+void ttyout_clear_flag(void)
+{
+    device_ready = false;
+}
+
+
+/******************************************************************************
+Description : Get the papertape device status.
+ Parameters : 
+    Returns : TRUE if device ready, else FALSE.
+   Comments : 
+ ******************************************************************************/
+bool ttyout_ready(void)
+{
+    return device_ready;
+}
+
+
+/******************************************************************************
+Description : Tick the state machine along a bit.
+ Parameters : cycles - number of imlac cycles that have elapsed
+    Returns : 
+   Comments : 
+ ******************************************************************************/
+void ttyout_tick(long cycles)
+{
+    /* if not open, no state change */
+    if (open_file == NULL)
+        return;
+
+    /* file mounted */
+    cycle_count -= cycles;
+    if (cycle_count <= 0L)
+    {
+        if (device_ready)
+        {
+            device_ready = false;
+            cycle_count += NOT_READY_CYCLES;
+        }
+        else
+        {
+            device_ready = true;
+            cycle_count += READY_CYCLES;
+        }
+    }
+}
diff --git a/vimlac/ttyout.h b/vimlac/ttyout.h
new file mode 100644
index 0000000..576dd64
--- /dev/null
+++ b/vimlac/ttyout.h
@@ -0,0 +1,17 @@
+/*
+ * Interface for the imlac TTYOUT (TTY input).
+ */
+
+#ifndef TTYOUT_H
+#define TTYOUT_H
+
+
+int ttyout_mount(char *fname);
+void ttyout_dismount(void);
+void ttyout_send(BYTE value);
+void ttyout_clear_flag(void);
+bool ttyout_ready(void);
+void ttyout_tick(long cycles);
+
+
+#endif