Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/main/java/com/loomcom/symon/machines/MulticompMachine.java

README.md

@@ -279,6 +279,10 @@ running.
   - Allow displaying ACIA status and dumping ACIA buffers, for
     debugging.
 
+  - CRTC emulation is very naive. The whole frame is drawn in one
+    CPU step. This should be improved by drawing scan lines during
+    machine steps to approximate real NTSC/PAL refresh rates.
+
   - Symbolic debugging.
 
 ## 7.0 Copyright and Acknowledgements

samples/ehbasic_crtc/.gitignore

@@ -0,0 +1,4 @@
+*.rom
+*.lst
+*.map
+*.o

samples/ehbasic_crtc/Makefile

@@ -0,0 +1,13 @@
+CA=ca65
+LD=ld65
+
+all: ehbasic
+
+ehbasic: ehbasic.o
+	$(LD) -C symon.config -vm -m ehbasic.map -o ehbasic.rom ehbasic.o
+
+ehbasic.o:
+	$(CA) --listing -o ehbasic.o min_mon.asm
+
+clean:
+	rm -f *.o *.rom *.map *.lst

samples/ehbasic_crtc/README.txt

@@ -0,0 +1,85 @@
+This directoy contains a very slightly modified version of EhBASIC 2.22
+to support the Symon simulator.
+
+Usage
+-----
+
+The pre-assemled ROM image 'ehbasic.rom' in the "samples" directory is all you
+need unless you want to rebuild this source code.
+
+Just copy the image 'ehbasic.rom' to the directory where you run Symon, and
+rename the file to 'rom.bin'. It will be loaded at memory location $D000 when
+the simulator starts up. Click "Run" and you'll be presented with BASIC.
+
+At the first prompt, type 'C' for a Cold Start
+
+When prompted for free memory, type: $C000
+
+Note that EhBASIC only accepts upper case input, so you'll need to use caps
+lock (the cruise control for cool) to really make the most of it.
+
+
+Building
+--------
+
+
+To build it, you'll need the CC65 tool chain from http://www.cc65.org/
+and a suitable version of 'make'. Just typing:
+
+   % make
+   
+in this directory should re-build everything. You'll get a listing,
+some object files, and the ROM image itself.
+
+
+Changes from the EhBASIC 2.22
+-----------------------------
+
+  - Minor syntax changes to allow assembly with the cc65 tool chain.
+
+  - Memory map modified for Symon.
+
+  - At reset, configure the 6551 ACIA for 8-N-1, 2400 baud.
+ 
+  - Monitor routines 'ACIAin' and 'ACIAout' modified for the 6551 ACIA.
+    Specifically, reading and writing will check the 6551 status register for
+    the status of rx and tx registers before receive or transmit.
+
+EhBASIC is copyright Lee Davison <leeedavison@googlemail.com>
+
+My changes are so slight that I hesitate to even say this, but for "CYA"
+reasons:
+
+Changes are copyright Seth Morabito <web@loomcom.com> and are distributed under
+the same license as EhBASIC. I claim no commercial interest whatsoever. Any
+commercial use must be negotiated directly with Lee Davison.
+
+
+Original EhBASIC 2.22 README
+----------------------------
+
+ Enhanced BASIC is a BASIC interpreter for the 6502 family microprocessors. It
+ is constructed to be quick and powerful and easily ported between 6502 systems.
+ It requires few resources to run and includes instructions to facilitate easy
+ low level handling of hardware devices. It also retains most of the powerful
+ high level instructions from similar BASICs.
+
+ EhBASIC is free but not copyright free. For non commercial use there is only one
+ restriction, any derivative work should include, in any binary image distributed,
+ the string "Derived from EhBASIC" and in any distribution that includes human
+ readable files a file that includes the above string in a human readable form
+ e.g. not as a comment in an HTML file.
+
+ For commercial use please contact me,  Lee Davison, at leeedavison@googlemail.com
+ for conditions.
+
+ For more information on EhBASIC, other versions of EhBASIC and other projects
+ please visit my site at ..
+
+	 http://mycorner.no-ip.org/index.html
+
+
+ P.S. c't magazin, henceforth refered to as "those thieving german bastards", are
+ prohibited from using this or any version of EhBASIC for any of their projects
+ or products. The excuse "we don't charge people for it" doesn't wash, it adds
+ value to your product so you owe me.

samples/ehbasic_crtc/min_mon.asm

@@ -0,0 +1,268 @@
+
+; minimal monitor for EhBASIC and 6502 simulator V1.05
+
+; To run EhBASIC on the simulator load and assemble [F7] this file, start the simulator
+; running [F6] then start the code with the RESET [CTRL][SHIFT]R. Just selecting RUN
+; will do nothing, you'll still have to do a reset to run the code.
+
+	.feature labels_without_colons
+	.include "basic.asm"
+
+; put the IRQ and MNI code in RAM so that it can be changed
+
+IRQ_vec	= VEC_SV+2		; IRQ code vector
+NMI_vec	= IRQ_vec+$0A	; NMI code vector
+
+; setup for the 6502 simulator environment
+
+IO_AREA		= $8800
+ACIAdata	= IO_AREA		; simulated ACIA r/w port
+ACIAstatus	= IO_AREA+1
+ACIAcommand	= IO_AREA+2
+ACIAcontrol	= IO_AREA+3
+
+	CPOS_L	= $E0		; Cursor address (low)
+	CPOS_H	= $E1		; Cursor address (high)
+	COUTC	= $E2		; Temp storage for char out.
+
+
+; now the code. all this does is set up the vectors and interrupt code
+; and wait for the user to select [C]old or [W]arm start. nothing else
+; fits in less than 128 bytes
+
+.segment "MONITOR"
+	.org	$FC00			; pretend this is in a 1/8K ROM
+
+; reset vector points here
+
+RES_vec
+	CLD				; clear decimal mode
+	LDX	#$FF			; empty stack
+	TXS				; set the stack
+
+	;; Initialize the CRTC
+	LDA	#$00		; Set cursor start to $7000
+	STA	CPOS_L
+	LDA	#$70
+	STA	CPOS_H
+
+	
+; Initialize the ACIA
+ACIA_init
+	LDA	#$00
+	STA	ACIAstatus		; Soft reset
+	LDA	#$0B
+	STA	ACIAcommand		; Parity disabled, IRQ disabled
+	LDA	#$1E
+	STA	ACIAcontrol		; Set output for 8-N-1 9600
+
+; set up vectors and interrupt code, copy them to page 2
+
+	LDY	#END_CODE-LAB_vec	; set index/count
+LAB_stlp
+	LDA	LAB_vec-1,Y		; get byte from interrupt code
+	STA	VEC_IN-1,Y		; save to RAM
+	DEY				; decrement index/count
+	BNE	LAB_stlp		; loop if more to do
+
+; now do the signon message, Y = $00 here
+
+LAB_signon
+	LDA	LAB_mess,Y		; get byte from sign on message
+	BEQ	LAB_nokey		; exit loop if done
+
+	JSR	V_OUTP		; output character
+	INY				; increment index
+	BNE	LAB_signon		; loop, branch always
+
+LAB_nokey
+	JSR	V_INPT		; call scan input device
+	BCC	LAB_nokey		; loop if no key
+
+	AND	#$DF			; mask xx0x xxxx, ensure upper case
+	CMP	#'W'			; compare with [W]arm start
+	BEQ	LAB_dowarm		; branch if [W]arm start
+
+	CMP	#'C'			; compare with [C]old start
+	BNE	RES_vec		; loop if not [C]old start
+
+	JMP	LAB_COLD		; do EhBASIC cold start
+
+LAB_dowarm
+	JMP	LAB_WARM		; do EhBASIC warm start
+
+
+ACIAout:	
+
+	PHA
+@loop:	LDA	ACIAstatus
+	AND	#$10
+	BEQ	@loop		; Wait for buffer to empty
+	PLA
+	STA	ACIAdata
+	RTS
+
+;;; Byte out to the CRTC
+;;;
+;;; 1. Increment cursor position.
+;;; 2. Scroll if necessary.
+;;; 3. Store new cursor position in CRTC.
+
+CRTCout:
+	STA	COUTC		; Store the character going out
+	JSR	ACIAout		; Also echo to terminal for debugging.
+
+	;; Is this a CR or LF?
+	CMP	#$0D
+	BEQ	@dolf
+	CMP	#$0A
+	BEQ	@docr
+	;; Is this a backspace?
+	CMP	#$08
+	BEQ	@dobs
+
+	;; Otherwise, this is a normal character
+	;; Do normal character stuff.
+	TYA			; Save Y
+	PHA
+	LDA	COUTC		; Restore character going out
+	LDY	#$00
+	STA	(CPOS_L),Y
+	INC	CPOS_L
+	PLA			; Restore Y
+	TAY
+	JMP	@done
+	
+
+@dolf:
+	;; Handle a LF
+	;;   Increment CRSR_R.
+	;;   If CRSR_R > ROW_M, scroll..
+	
+
+@docr:
+	;; Handle a CR
+	;;   Set CRSR_C = 0.
+
+@dobs:	
+	;; 2. Is this a backspace?
+	;;   IF CRSR_C == 0 && CRSR_R == 0
+	;;     Do nothing, already home
+	;;   If CRSR_C > 0
+	;;     Set CRSR_C = CRSR_C - 1
+	;;   Else
+	;;     Set CRSR_C = COL_M
+	;;     Set CRSR_R = CRSR_R - 1
+	;;
+
+@done:
+	;; Now new cursor Column and Row have been calculated,
+	;; and we know if we need to scroll.
+	;;
+	;; (CPOS_H) = $70 + (row / 
+	
+	RTS
+
+	;; Convert CRSR_C, CRSR_R into Cursor Address.
+	;; Result stored in CPOS_L,CPOS_H
+	;; 
+;; C_POS:	
+;; 	;; Multiply CRSR_R * $28, store result in CPOS_L,CPOS_H
+;; 	LDA	CRSR_R		; Copy CRSR_R to CPOS_L
+;; 	STA	CPOS_L
+;; 	LDA	#$00
+;; 	LDX	#$08
+;; 	LSR	CPOS_L
+;; @cpos1:	BCC	@cpos2
+;; 	CLC
+;; 	ADC	#$28
+;; @cpos2:	ROR
+;; 	ROR	CPOS_L
+;; 	DEX
+;; 	BNE	@cpos1
+;; 	STA	CPOS_H
+;; 	;; At this point, CPOS_L,CPOS_H holds the product
+;; 	;; of CRSR_R and $28. We need to add CRSR_C with
+;; 	;; an unsigned 16-bit add.
+;; 	CLC
+;; 	LDA	CRSR_C
+;; 	ADC	CPOS_L
+;; 	STA	CPOS_L
+;; 	BCC	@noinc		; If carry wasn't set, no need
+;; 	INC	CPOS_H		;    to increment CPOS_H
+;; @noinc:	
+	
+;
+; byte in from ACIA. This subroutine will also force
+; all lowercase letters to be uppercase.
+;
+ACIAin
+	LDA	ACIAstatus		; Read 6551 status
+	AND	#$08			;
+	BEQ	LAB_nobyw		; If rx buffer empty, no byte
+
+	LDA	ACIAdata		; Read byte from 6551
+	CMP	#'a'			; Is it < 'a'?
+	BCC	@done			; Yes, we're done
+	CMP	#'{'			; Is it >= '{'?
+	BCS	@done			; Yes, we're done
+	AND	#$5f			; Otherwise, mask to uppercase
+@done
+	SEC				; Flag byte received
+	RTS
+
+LAB_nobyw
+	CLC				; flag no byte received
+no_load				; empty load vector for EhBASIC
+no_save				; empty save vector for EhBASIC
+	RTS
+
+; vector tables
+
+LAB_vec
+	.word	ACIAin		; byte in from simulated ACIA
+	.word	CRTCout		; byte out to simulated ACIA
+	.word	no_load		; null load vector for EhBASIC
+	.word	no_save		; null save vector for EhBASIC
+
+; EhBASIC IRQ support
+
+IRQ_CODE
+	PHA				; save A
+	LDA	IrqBase		; get the IRQ flag byte
+	LSR				; shift the set b7 to b6, and on down ...
+	ORA	IrqBase		; OR the original back in
+	STA	IrqBase		; save the new IRQ flag byte
+	PLA				; restore A
+	RTI
+
+; EhBASIC NMI support
+
+NMI_CODE
+	PHA				; save A
+	LDA	NmiBase		; get the NMI flag byte
+	LSR				; shift the set b7 to b6, and on down ...
+	ORA	NmiBase		; OR the original back in
+	STA	NmiBase		; save the new NMI flag byte
+	PLA				; restore A
+	RTI
+
+END_CODE
+
+; sign on string
+
+LAB_mess
+	.byte	$0D,$0A,"Symon (c) 2008-2014, Seth Morabito"
+	.byte   $0D,$0A,"Enhanced 6502 BASIC 2.22 (c) Lee Davison"
+	.byte   $0D,$0A,"[C]old/[W]arm ?",$00
+
+
+; system vectors
+
+.segment "VECTORS"
+	.org	$FFFA
+
+	.word	NMI_vec		; NMI vector
+	.word	RES_vec		; RESET vector
+	.word	IRQ_vec		; IRQ vector
+

samples/ehbasic_crtc/symon.config

@@ -0,0 +1,18 @@
+MEMORY {
+RAM1: start = $0000, size = $8000;
+ROM1: start = $C000, size = $3C00, fill = yes;
+MONITOR: start = $FC00, size = $3FA, fill = yes;
+ROMV: start = $FFFA, size = $6, file = %O, fill = yes;
+}
+
+SEGMENTS {
+CODE:     load = ROM1, type = ro;
+DATA:     load = ROM1, type = ro;
+MONITOR:  load = MONITOR, type = ro;
+VECTORS:  load = ROMV, type = ro;
+}
+
+SYMBOLS {
+__STACKSIZE__ = $0300;
+}
+

src/main/java/com/loomcom/symon/Simulator.java

@@ -364,6 +364,9 @@ public class Simulator {
 
         if (videoWindow != null && stepsSinceLastCrtcRefresh++ > stepsBetweenCrtcRefreshes) {
             stepsSinceLastCrtcRefresh = 0;
+            if (videoWindow.isVisible()) {
+                videoWindow.repaint();
+            }
         }
 
         // This is a very expensive update, and we're doing it without

src/main/java/com/loomcom/symon/devices/Crtc.java

@@ -65,6 +65,11 @@ public class Crtc extends Device {
 
     private int currentRegister = 0;
 
+    // Status bits
+    private boolean rowColumnAddressing = false;
+    private boolean displayEnableSkew = false;
+    private boolean cursorSkew = false;
+
     private Memory memory;
 
     public Crtc(int deviceAddress, Memory memory) throws MemoryRangeException, IOException {
@@ -118,8 +123,9 @@ public class Crtc extends Device {
         return null;
     }
 
-    public int[] getDmaAccess() {
+    public int getCharAtAddress(int address) throws MemoryAccessException {
-        return memory.getDmaAccess();
+        // TODO: Row/Column addressing
+        return memory.read(address);
     }
 
     public int getHorizontalDisplayed() {
@@ -162,6 +168,18 @@ public class Crtc extends Device {
         return pageSize;
     }
 
+    public boolean getRowColumnAddressing() {
+        return rowColumnAddressing;
+    }
+
+    public boolean getDisplayEnableSkew() {
+        return displayEnableSkew;
+    }
+
+    public boolean getCursorSkew() {
+        return cursorSkew;
+    }
+
     private void setCurrentRegister(int registerNumber) {
         this.currentRegister = registerNumber;
     }
@@ -180,7 +198,9 @@ public class Crtc extends Device {
                 pageSize = horizontalDisplayed * verticalDisplayed;
                 break;
             case MODE_CONTROL:
-                // TODO: Implement multiple addressing modes and cursor skew.
+                rowColumnAddressing = (data & 0x04) != 0;
+                displayEnableSkew = (data & 0x10) != 0;
+                cursorSkew = (data & 0x20) != 0;
                 break;
             case SCAN_LINE:
                 scanLinesPerRow = data;

src/main/java/com/loomcom/symon/devices/Pia.java

@@ -23,45 +23,19 @@
 
 package com.loomcom.symon.devices;
 
-import com.loomcom.symon.exceptions.MemoryAccessException;
 import com.loomcom.symon.exceptions.MemoryRangeException;
 
-public class Via extends Device {
+public abstract class Pia extends Device {
-    public static final int VIA_SIZE = 16;
 
-    private static final int ORB = 0;
+    private final String name;
-    private static final int ORA = 1;
-    private static final int DDRB = 2;
-    private static final int DDRA = 3;
-    private static final int T1C_L = 4;
-    private static final int T1C_H = 5;
-    private static final int T1L_L = 6;
-    private static final int T1L_H = 7;
-    private static final int T2C_L = 8;
-    private static final int T2C_H = 9;
-    private static final int SR = 10;
-    private static final int ACR = 11;
-    private static final int PCR = 12;
-    private static final int IFR = 13;
-    private static final int IER = 14;
-    private static final int ORA_H = 15;
 
-    public Via(int address) throws MemoryRangeException {
+    public Pia(int startAddress, int endAddress, String name) throws MemoryRangeException {
-      super(address, address + VIA_SIZE - 1, "VIA");
+        super(startAddress, endAddress, name);
-    }
+        this.name = name;
-
-    @Override
-    public void write(int address, int data) throws MemoryAccessException {
-
-    }
-
-    @Override
-    public int read(int address) throws MemoryAccessException {
-        return 0;
     }
 
     @Override
     public String toString() {
-        return null;
+        return name;
     }
 }

src/main/java/com/loomcom/symon/devices/Via6522.java

@@ -0,0 +1,90 @@
+package com.loomcom.symon.devices;
+
+import com.loomcom.symon.exceptions.MemoryAccessException;
+import com.loomcom.symon.exceptions.MemoryRangeException;
+
+/**
+ * Very basic implementation of a MOS 6522 VIA.
+ *
+ * TODO: Implement timers as threads.
+ */
+public class Via6522 extends Pia {
+    public static final int VIA_SIZE = 16;
+
+    enum Register {
+        ORB, ORA, DDRB, DDRA, T1C_L, T1C_H, T1L_L, T1L_H,
+        T2C_L, T2C_H, SR, ACR, PCR, IFR, IER, ORA_H
+    }
+
+    // Ports A and B
+    private char[] portData = {0, 0};
+    private char[] portDirections = {0, 0};
+
+    public Via6522(int address) throws MemoryRangeException {
+        super(address, address + VIA_SIZE - 1, "MOS 6522 VIA");
+    }
+
+    @Override
+    public void write(int address, int data) throws MemoryAccessException {
+        Register[] registers = Register.values();
+
+        if (address >= registers.length) {
+            throw new MemoryAccessException("Unknown register: " + address);
+        }
+
+        Register r = registers[address];
+
+        switch (r) {
+            case ORA:
+            case ORB:
+            case DDRA:
+            case DDRB:
+            case T1C_L:
+            case T1C_H:
+            case T1L_L:
+            case T1L_H:
+            case T2C_L:
+            case T2C_H:
+            case SR:
+            case ACR:
+            case PCR:
+            case IFR:
+            case IER:
+            case ORA_H:
+            default:
+        }
+    }
+
+    @Override
+    public int read(int address) throws MemoryAccessException {
+        Register[] registers = Register.values();
+
+        if (address >= registers.length) {
+            throw new MemoryAccessException("Unknown register: " + address);
+        }
+
+        Register r = registers[address];
+
+        switch (r) {
+            case ORA:
+            case ORB:
+            case DDRA:
+            case DDRB:
+            case T1C_L:
+            case T1C_H:
+            case T1L_L:
+            case T1L_H:
+            case T2C_L:
+            case T2C_H:
+            case SR:
+            case ACR:
+            case PCR:
+            case IFR:
+            case IER:
+            case ORA_H:
+            default:
+        }
+
+        return 0;
+    }
+}

src/main/java/com/loomcom/symon/machines/Machine.java

@@ -29,7 +29,7 @@ import com.loomcom.symon.Cpu;
 import com.loomcom.symon.devices.Acia;
 import com.loomcom.symon.devices.Crtc;
 import com.loomcom.symon.devices.Memory;
-import com.loomcom.symon.devices.Via;
+import com.loomcom.symon.devices.Pia;
 import com.loomcom.symon.exceptions.MemoryRangeException;
 
 
@@ -43,7 +43,7 @@ public interface Machine {
     
     public Acia getAcia();
     
-    public Via getVia();
+    public Pia getPia();
     
     public Crtc getCrtc();
     

src/main/java/com/loomcom/symon/machines/MulticompMachine.java

@@ -30,8 +30,8 @@ import com.loomcom.symon.devices.Acia;
 import com.loomcom.symon.devices.Acia6850;
 import com.loomcom.symon.devices.Crtc;
 import com.loomcom.symon.devices.Memory;
+import com.loomcom.symon.devices.Pia;
 import com.loomcom.symon.devices.SdController;
-import com.loomcom.symon.devices.Via;
 import com.loomcom.symon.exceptions.MemoryRangeException;
 import java.io.File;
 import java.util.logging.Logger;
@@ -118,7 +118,7 @@ public class MulticompMachine implements Machine {
     }
 
     @Override
-    public Via getVia() {
+    public Pia getPia() {
         return null;
     }
 

src/main/java/com/loomcom/symon/machines/SimpleMachine.java

@@ -5,7 +5,7 @@ import com.loomcom.symon.Cpu;
 import com.loomcom.symon.devices.Acia;
 import com.loomcom.symon.devices.Crtc;
 import com.loomcom.symon.devices.Memory;
-import com.loomcom.symon.devices.Via;
+import com.loomcom.symon.devices.Pia;
 import com.loomcom.symon.exceptions.MemoryRangeException;
 
 /**
@@ -52,7 +52,7 @@ public class SimpleMachine implements Machine {
     }
 
     @Override
-    public Via getVia() {
+    public Pia getPia() {
         return null;
     }
 

src/main/java/com/loomcom/symon/machines/SymonMachine.java

@@ -26,11 +26,7 @@ package com.loomcom.symon.machines;
 
 import com.loomcom.symon.Bus;
 import com.loomcom.symon.Cpu;
-import com.loomcom.symon.devices.Acia;
+import com.loomcom.symon.devices.*;
-import com.loomcom.symon.devices.Acia6551;
-import com.loomcom.symon.devices.Crtc;
-import com.loomcom.symon.devices.Memory;
-import com.loomcom.symon.devices.Via;
 import com.loomcom.symon.exceptions.MemoryRangeException;
 import java.io.File;
 import java.util.logging.Logger;
@@ -48,16 +44,15 @@ public class SymonMachine implements Machine {
     private static final int MEMORY_BASE = 0x0000;
     private static final int MEMORY_SIZE = 0x8000;
 
-    // VIA at $8000-$800F
+    // PIA at $8000-$800F
 
-    private static final int VIA_BASE = 0x8000;
+    private static final int PIA_BASE = 0x8000;
 
     // ACIA at $8800-$8803
     private static final int ACIA_BASE = 0x8800;
 
     // CRTC at $9000-$9001
     private static final int CRTC_BASE = 0x9000;
-    private static final int VIDEO_RAM_BASE = 0x7000;
 
     // 16KB ROM at $C000-$FFFF
     private static final int ROM_BASE = 0xC000;
@@ -68,7 +63,7 @@ public class SymonMachine implements Machine {
     private final Bus    bus;
     private final Cpu    cpu;
     private final Acia   acia;
-    private final Via    via;
+    private final Pia    pia;
     private final Crtc   crtc;
     private final Memory ram;
     private       Memory rom;
@@ -78,13 +73,13 @@ public class SymonMachine implements Machine {
         this.bus = new Bus(BUS_BOTTOM, BUS_TOP);
         this.cpu = new Cpu();
         this.ram = new Memory(MEMORY_BASE, MEMORY_BASE + MEMORY_SIZE - 1, false);
-        this.via = new Via(VIA_BASE);
+        this.pia = new Via6522(PIA_BASE);
         this.acia = new Acia6551(ACIA_BASE);
         this.crtc = new Crtc(CRTC_BASE, ram);
 
         bus.addCpu(cpu);
         bus.addDevice(ram);
-        bus.addDevice(via);
+        bus.addDevice(pia);
         bus.addDevice(acia);
         bus.addDevice(crtc);
         
@@ -124,8 +119,8 @@ public class SymonMachine implements Machine {
     }
 
     @Override
-    public Via getVia() {
+    public Pia getPia() {
-        return via;
+        return pia;
     }
 
     @Override

src/main/java/com/loomcom/symon/ui/VideoWindow.java

@@ -25,6 +25,7 @@ package com.loomcom.symon.ui;
 
 import com.loomcom.symon.devices.Crtc;
 import com.loomcom.symon.devices.DeviceChangeListener;
+import com.loomcom.symon.exceptions.MemoryAccessException;
 
 import javax.swing.*;
 import java.awt.*;
@@ -36,8 +37,11 @@ import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
+import java.util.logging.Level;
 import java.util.logging.Logger;
 
+import static java.lang.System.*;
+
 /**
  * VideoWindow represents a graphics framebuffer backed by a 6545 CRTC.
  * Each time the window's VideoPanel is repainted, the video memory is
@@ -65,7 +69,6 @@ public class VideoWindow extends JFrame implements DeviceChangeListener {
 
     private BufferedImage image;
     private int[] charRom;
-    private int[] videoRam;
 
     private int horizontalDisplayed;
     private int verticalDisplayed;
@@ -85,17 +88,21 @@ public class VideoWindow extends JFrame implements DeviceChangeListener {
     private class VideoPanel extends JPanel {
         @Override
         public void paintComponent(Graphics g) {
-            for (int i = 0; i < crtc.getPageSize(); i++) {
+            try {
-                int address = crtc.getStartAddress() + i;
+                for (int i = 0; i < crtc.getPageSize(); i++) {
-                int originX = (i % horizontalDisplayed) * CHAR_WIDTH;
+                    int address = crtc.getStartAddress() + i;
-                int originY = (i / horizontalDisplayed) * scanLinesPerRow;
+                    int originX = (i % horizontalDisplayed) * CHAR_WIDTH;
-                image.getRaster().setPixels(originX, originY, CHAR_WIDTH, scanLinesPerRow, getGlyph(address));
+                    int originY = (i / horizontalDisplayed) * scanLinesPerRow;
+                    image.getRaster().setPixels(originX, originY, CHAR_WIDTH, scanLinesPerRow, getGlyph(address));
+                }
+                Graphics2D g2d = (Graphics2D) g;
+                if (shouldScale) {
+                    g2d.scale(scaleX, scaleY);
+                }
+                g2d.drawImage(image, 0, 0, null);
+            } catch (MemoryAccessException ex) {
+                logger.log(Level.SEVERE, "Memory Access Exception, can't paint video window! " + ex.getMessage());
             }
-            Graphics2D g2d = (Graphics2D)g;
-            if (shouldScale) {
-                g2d.scale(scaleX, scaleY);
-            }
-            g2d.drawImage(image, 0, 0, null);
         }
 
         @Override
@@ -131,8 +138,7 @@ public class VideoWindow extends JFrame implements DeviceChangeListener {
 
         this.scheduler = Executors.newSingleThreadScheduledExecutor();
         this.crtc = crtc;
-        this.charRom = loadCharRom("/pet.rom");
+        this.charRom = loadCharRom("/cga8.rom");
-        this.videoRam = crtc.getDmaAccess();
         this.scaleX = scaleX;
         this.scaleY = scaleY;
         this.shouldScale = (scaleX > 1 || scaleY > 1);
@@ -158,13 +164,6 @@ public class VideoWindow extends JFrame implements DeviceChangeListener {
 
     }
 
-    public void refreshDisplay() {
-        // TODO: Verify whether this is necessary. Does `repaint()' do anything if the window is not visible?
-        if (isVisible()) {
-            repaint();
-        }
-    }
-
     /**
      * Called by the CRTC on state change.
      */
@@ -239,15 +238,13 @@ public class VideoWindow extends JFrame implements DeviceChangeListener {
      * @param address The address of the character being requested.
      * @return An array of integers representing the pixel data.
      */
-    private int[] getGlyph(int address) {
+    private int[] getGlyph(int address) throws MemoryAccessException {
-        int chr = videoRam[address];
+        int chr = crtc.getCharAtAddress(address);
         int romOffset = (chr & 0xff) * (CHAR_HEIGHT * CHAR_WIDTH);
         int[] glyph = new int[CHAR_WIDTH * scanLinesPerRow];
 
         // Populate the character
-        for (int i = 0; i < (CHAR_WIDTH * Math.min(CHAR_HEIGHT, scanLinesPerRow)); i++) {
+        arraycopy(charRom, romOffset, glyph, 0, CHAR_WIDTH * Math.min(CHAR_HEIGHT, scanLinesPerRow));
-            glyph[i] = charRom[romOffset + i];
-        }
 
         // Overlay the cursor
         if (!hideCursor && crtc.isCursorEnabled() && crtc.getCursorPosition() == address) {

src/test/java/com/loomcom/symon/CrtcTest.java

@@ -19,19 +19,17 @@ import static org.mockito.Mockito.*;
 public class CrtcTest {
 
     Crtc crtc;
+    Memory memory;
 
     @Mock
     DeviceChangeListener changeListener;
-    @Mock
-    Memory memory;
 
     @Before
     public void createDevices() throws Exception {
+        memory = new Memory(0, 0x7fff);
+
         crtc = new Crtc(0x9000, memory);
         crtc.registerListener(changeListener);
-
-        when(memory.startAddress()).thenReturn(0);
-        when(memory.endAddress()).thenReturn(0x7fff);
     }
 
     @Test
@@ -340,4 +338,58 @@ public class CrtcTest {
         crtc.write(1, 0x80); // Can't position cursor
     }
 
+    @Test
+    public void shouldSetRowColumnAddressing() throws Exception {
+        assertEquals(false, crtc.getRowColumnAddressing());
+        crtc.write(0, 8);        // Select mode control register
+        crtc.write(1, 0x04);
+        assertEquals(true, crtc.getRowColumnAddressing());
+    }
+
+    @Test
+    public void shouldSetDisplayEnableSkew() throws Exception {
+        assertEquals(false, crtc.getDisplayEnableSkew());
+        crtc.write(0, 8);        // Select mode control register
+        crtc.write(1, 0x10);
+        assertEquals(true, crtc.getDisplayEnableSkew());
+    }
+
+    @Test
+    public void shouldSetCursorSkew() throws Exception {
+        assertEquals(false, crtc.getCursorSkew());
+        crtc.write(0, 8);        // Select mode control register
+        crtc.write(1, 0x20);
+        assertEquals(true, crtc.getCursorSkew());
+    }
+
+    @Test
+    public void shouldDoStraightBinaryAddressing() throws Exception {
+        crtc.write(0, 8);
+        crtc.write(1, 0); // Select straight binary addressing
+
+        // Fill the memory with a repeating pattern
+        int videoMemoryBase = 0x7000;
+        int j = 0;
+
+        for (int i = 0; i < 2048; i++) {
+            memory.write(videoMemoryBase + i, j);
+            if (j == 255) {
+                j = 0;
+            } else {
+                j++;
+            }
+        }
+
+        // Now verify that straight-binary addressing of the CRTC works
+        j = 0;
+
+        for (int i = 0; i < 2048; i++) {
+            assertEquals(j, crtc.getCharAtAddress(videoMemoryBase + i));
+            if (j == 255) {
+                j = 0;
+            } else {
+                j++;
+            }
+        }
+    }
 }