Advanced Assembly Programming for the Apple IIsedwards/2020-vcfw-6502/2020...opcode mode 0 1...

Advanced AssemblyProgrammingfor the Apple II

Stephen A. Edwards

6502 image from https://www.pagetable.com/?p=1295

DATA BUS ADDR.

ADDRESS BUS

40 21222324252627282930313233343536373839

1 201918171615141312111098765432

VSSA12A13A14A15D7D6D5D4D3D2D1D0

VSS A11A10A9A8A7A6A5A4A3A2A1A0VCCSYNCRDY(OUT)1 IRQ NMI

R/W02(OUT)

After Bill Bertram, Wikipedia

The 6502 Programmer’s Model

07A ACCUMULATOR

Y INDEX REGISTER

X INDEX REGISTER

15PCLPCH PROGRAM COUNTER

S1 STACK POINTER

STATUS REGISTERC

Interrupt Disable

Decimal Mode

Overflow

Negative

07A ACCUMULATOR

Y INDEX REGISTER

X INDEX REGISTER

S1 STACK POINTER

STATUS REGISTERC

Interrupt Disable

Decimal Mode

Overflow

Negative

07A ACCUMULATOR

Y INDEX REGISTER

X INDEX REGISTER

S1 STACK POINTER

STATUS REGISTERC

Interrupt Disable

Decimal Mode

Overflow

Negative

07A ACCUMULATOR

Y INDEX REGISTER

X INDEX REGISTER

S1 STACK POINTER

STATUS REGISTERC

Interrupt Disable

Decimal Mode

Overflow

Negative

07A ACCUMULATOR

Y INDEX REGISTER

X INDEX REGISTER

S1 STACK POINTER

STATUS REGISTERC

Interrupt Disable

Decimal Mode

Overflow

Negative

07A ACCUMULATOR

Y INDEX REGISTER

X INDEX REGISTER

S1 STACK POINTER

STATUS REGISTERC

Interrupt Disable

Decimal Mode

Overflow

Negative

isual6502.o

6502 Instruction Encoding

groupmodeopcode

groupmodeopcode literal or address

groupmodeopcode low address byte high address byte

10modeopcode “Group one” add, compare; most addressing modes

01modeopcode “Group two” shift/rotate, load/store X; fewer modes

001modeopcode Load/store Y, compare X & Y

0xy01op1 Index register instructions

00011flag Flag set/clear

00001vflag Branches

0 op 0 op 0 0 0 Stack instructions

11 Unused in the 6502

00001vflag Branches

Group One Instructions 10modeopcode

A9 42 LDA #$42 ;Load Accumulator Immediate

#$4242

A9 42 LDA #$42 ;Load Accumulator Immediate

#$4242

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page

42STA $42

Zero PageMemory

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page

42STA $42

Zero PageMemory

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by X

$42,XADC

Zero PageMemory

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by X

$42,XADC

$42 + X

Zero PageMemory

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry Absolute

Memory

$420101 42

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry Absolute

Memory

$420101 42

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry AbsoluteDD 05 42 CMP $4205,X ;Compare Absolute Indexed by X

Memory

$420505 42

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry AbsoluteDD 05 42 CMP $4205,X ;Compare Absolute Indexed by X

$4205 + X

Memory

$420505 42

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry AbsoluteDD 05 42 CMP $4205,X ;Compare Absolute Indexed by X39 06 42 AND $4206,Y ;Logical AND Absolute Indexed by Y

Memory

$4206,Y

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry AbsoluteDD 05 42 CMP $4205,X ;Compare Absolute Indexed by X39 06 42 AND $4206,Y ;Logical AND Absolute Indexed by Y

$4206 + Y

Memory

+$4206,Y

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry AbsoluteDD 05 42 CMP $4205,X ;Compare Absolute Indexed by X39 06 42 AND $4206,Y ;Logical AND Absolute Indexed by Y11 42 ORA ($42),Y ;Logical OR Indirect Indexed

($42),Y

Memory

$42$43

Zero PageMemory

($42),Y

Memory

$42$43

Zero PageMemory

($42),Y

Memory

A9 42 LDA #$42 ;Load Accumulator Immediate85 42 STA $42 ;Store Accumulator Zero Page75 42 ADC $42,X ;Add with Carry Zero Page Indexed by XED 01 42 SBC $4201 ;Subtract w/Carry AbsoluteDD 05 42 CMP $4205,X ;Compare Absolute Indexed by X39 06 42 AND $4206,Y ;Logical AND Absolute Indexed by Y11 42 ORA ($42),Y ;Logical OR Indirect Indexed41 42 EOR ($42,X) ;Exclusive OR Indexed Indirect

Memory

($42,X)

$42 + X$43 + X

Zero PageMemory

Memory

($42,X)

$42 + X$43 + X

Zero PageMemory

Memory

($42,X)

Apple II ZeroPage Usage

Monitor

ApplesoftDOS 3.3 ProDOS

Source: comp.sys.apple2 FAQ, Wagner,Wirth and Lechner

Monitor

Applesoft

DOS 3.3 ProDOS

Monitor ApplesoftDOS 3.3 ProDOS

Group Two Instructions 01modeopcode

A2 42 LDX #$42 ;Load X Immediate

#$4242

A2 42 LDX #$42 ;Load X Immediate86 42 STX $42 ;Store X Zero Page

86STX $42

Zero PageMemory

A2 42 LDX #$42 ;Load X Immediate86 42 STX $42 ;Store X Zero PageF6 42 INC $42,X ;Increment Zero Page Indexed by X

$42 + X

Zero PageMemory

A2 42 LDX #$42 ;Load X Immediate86 42 STX $42 ;Store X Zero PageF6 42 INC $42,X ;Increment Zero Page Indexed by XCE 77 42 DEC $4277 ;Decrement Absolute

$427777 42

Memory

A2 42 LDX #$42 ;Load X Immediate86 42 STX $42 ;Store X Zero PageF6 42 INC $42,X ;Increment Zero Page Indexed by XCE 77 42 DEC $4277 ;Decrement Absolute0A ASL ;Arithmetic Shift Left Accumulator

A2 42 LDX #$42 ;Load X Immediate86 42 STX $42 ;Store X Zero PageF6 42 INC $42,X ;Increment Zero Page Indexed by XCE 77 42 DEC $4277 ;Decrement Absolute0A ASL ;Arithmetic Shift Left Accumulator5E 80 42 LSR $4280,X ;Logical Shift Right Absolute Indexed by X

5ELSR $4280,X

A2 42 LDX #$42 ;Load X Immediate86 42 STX $42 ;Store X Zero PageF6 42 INC $42,X ;Increment Zero Page Indexed by XCE 77 42 DEC $4277 ;Decrement Absolute0A ASL ;Arithmetic Shift Left Accumulator5E 80 42 LSR $4280,X ;Logical Shift Right Absolute Indexed by X2A ROL ;Rotate Left Accumulator

A2 42 LDX #$42 ;Load X Immediate86 42 STX $42 ;Store X Zero PageF6 42 INC $42,X ;Increment Zero Page Indexed by XCE 77 42 DEC $4277 ;Decrement Absolute0A ASL ;Arithmetic Shift Left Accumulator5E 80 42 LSR $4280,X ;Logical Shift Right Absolute Indexed by X2A ROL ;Rotate Left Accumulator6A ROR ;Rotate Right Accumulator

Other Instructions 001modeopcode

A0 42 LDY #$42 ;Load Y Immediate94 42 STY $42,X ;Store Y Zero Page Indexed by XC4 42 CPY $42 ;Compare Y Zero PageEC 50 42 CPX $4250 ;Compare X Absolute

A0 42 LDY #$42 ;Load Y Immediate94 42 STY $42,X ;Store Y Zero Page Indexed by XC4 42 CPY $42 ;Compare Y Zero PageEC 50 42 CPX $4250 ;Compare X Absolute4C 40 42 JMP $4240 ;Jump Absolute

4CJMP $4240

PCH PCL

A0 42 LDY #$42 ;Load Y Immediate94 42 STY $42,X ;Store Y Zero Page Indexed by XC4 42 CPY $42 ;Compare Y Zero PageEC 50 42 CPX $4250 ;Compare X Absolute4C 40 42 JMP $4240 ;Jump Absolute6C 41 42 JMP ($4241) ;Jump Indirect

6CJMP $4241

PCH PCL

$4241$4242

Memory

A0 42 LDY #$42 ;Load Y Immediate94 42 STY $42,X ;Store Y Zero Page Indexed by XC4 42 CPY $42 ;Compare Y Zero PageEC 50 42 CPX $4250 ;Compare X Absolute4C 40 42 JMP $4240 ;Jump Absolute6C 41 42 JMP ($4241) ;Jump Indirect

6CJMP $4241

PCH PCL

$4241$4242

Memory

A0 42 LDY #$42 ;Load Y Immediate94 42 STY $42,X ;Store Y Zero Page Indexed by XC4 42 CPY $42 ;Compare Y Zero PageEC 50 42 CPX $4250 ;Compare X Absolute4C 40 42 JMP $4240 ;Jump Absolute6C 41 42 JMP ($4241) ;Jump Indirect2C 57 42 BIT $4257 ;Test Bits w/ Accum. Absolute

2CBIT $4257

Memory

($4257) ∧ A→ Z, ($4257)→ N, V, –, –, –, –, –, –

The byte at address $4257 is read and bits 7 and 6 are stored in flags N and V.The value is ANDed with the accumulator and the Z flag is set on a zero result

The most bizarre of 6502 instructions: CMP with logical AND instead ofsubtraction, but also stores the MSB and 6SB into N and V.

2CBIT $4257

Memory

2CBIT $4257

ANDA $4257

Memory

Single-Byte Data Instructions 0xy01op1

E8 INX ;Increment XCA DEX ;Decrement XC8 INY ;Increment Y88 DEY ;Decrement YAA TAX ;Transfer Accumulator to XA8 TAY ;Transfer Accumulator to Y8A TXA ;Transfer X to Accumulator98 TYA ;Transfer Y to AccumulatorBA TSX ;Transfer Stack Pointer to X9A TXS ;Transfer X to Stack Pointer

Single-Byte Flag Instructions 00011flag

18 CLC ;Clear Carry Flag38 SEC ;Set Carry Flag58 CLI ;Clear Interrupt Disable78 SEI ;Set Interrupt DisableB8 CLV ;Clear Overflow Flag

Set and clear the carry flag; useful with ADC and SBC

Enable and disable interrupts. Useful for critical regions

Clear the overflow flag

Single-Byte Flag Instructions 00011flag

18 CLC ;Clear Carry Flag38 SEC ;Set Carry Flag58 CLI ;Clear Interrupt Disable78 SEI ;Set Interrupt DisableB8 CLV ;Clear Overflow FlagD8 CLD ;Clear Decimal Mode (ADC, SBC perform binary arithmetic)F8 SED ;Set Decimal Mode (ADC, SBC perform BCD arithemtic)

In decimal mode, ADC and SBCperform BCD arithmetic.

In decimal mode, $19 + $1 = $20

In binary mode, $19 + $1 = $1A

US Patent 3,991,307 (1976) describes the 6502’s decimal adjust logic

The 6502-based Ricoh 2A03 used in the Famicom/Nintendo EntertainmentSystem does not support decimal mode

Branch Instructions 00001vflag

10 FE START BPL START ;Branch on Plus N=030 0A BMI END ;Branch on Minus N=150 FA BVC START ;Branch on Overflow Clear V=070 06 BVS END ;Branch on Overflow Set V=190 F6 BCC START ;Branch on Carry Clear C=0B0 02 BCS END ;Branch on Carry Set C=1D0 F2 BNE START ;Branch on Not Equal Z=0F0 FE END BEQ END ;Branch on Equal Z=1

Conditionally branch depending on the state of one of the four flags

Branch destination is program-counter-relative: between −128 and 127 bytes

Use JMP to branch farther

BNE and BEQ could be called “branch on non-zero” and “branch on zero”

The 65C02 added the unconditional BRA instruction

Stack Instructions 0 op 0 op 0 0 0

20 ED FD JSR $FDED ;Jump to Subroutine60 RTS ;Return from Subroutine

For subroutine linkage:

JSR $FDED pushes the program counter onto the stack then jumps to $FDED

RTS pops the program counter value off the stack, returning to just after theJSR that sent it there

20 ED FD JSR $FDED ;Jump to Subroutine60 RTS ;Return from Subroutine40 RTI ;Return from Interrupt

Return from an interrupt by popping the status register and the programcounter from the stack. (Like RTS, but also restores the status register)

Useful in interrupt routines and returning from BRK

20 ED FD JSR $FDED ;Jump to Subroutine60 RTS ;Return from Subroutine40 RTI ;Return from Interrupt08 PHP ;Push Processor Status28 PLP ;Pull Processor Status48 PHA ;Push Accumulator68 PLA ;Pull Accumulator

Save and restore the processor status on the stack

Save and restore the accumulator on the stack

Typical processors encourage functions to save registers on the stack, but the6502’s cramped (256 byte) stack demands parsimony

20 ED FD JSR $FDED ;Jump to Subroutine60 RTS ;Return from Subroutine40 RTI ;Return from Interrupt08 PHP ;Push Processor Status28 PLP ;Pull Processor Status48 PHA ;Push Accumulator68 PLA ;Pull Accumulator00 BRK ;BreakEA NOP ;No operation

BRK is a software interrupt that pushes the current program counter andstatus register on the stack before jumping to the break vector stored (inROM) at $FFFE.

NOP does nothing for two cycles. Useful in delay loops or to patch a programin memory.

Hello World in 6502 Assembly

ORG $8000 ; Set code origin

COUT EQU $FDED ; Define COUT label (character out)

ENTRY LDX #0 ; Use X because COUT leaves it unchangedLOOP LDA MSG,X ; Get character from string

BEQ DONE ; Are we at the end?JSR COUT ; No: print the characterINX ; Go to next character in stringBNE LOOP ; A trick: always taken

DONE RTS ; Return

MSG ASC "Hello World!" ; Generate ASCII character codesDB $8D,00 ; Define byte: 8D is return, 00 is end

Printed Assembler Output1 ORG $8000 ; Set code origin23 COUT EQU $FDED ; Define COUT label (character out)4

8000: A2 00 5 ENTRY LDX #0 ; Use X because COUT leaves it unchanged8002: BD 0E 80 6 LOOP LDA MSG,X ; Get character from string8005: F0 06 7 BEQ DONE ; Are we at the end?8007: 20 ED FD 8 JSR COUT ; No: print the character800A: E8 9 INX ; Go to next character in string800B: D0 F5 10 BNE LOOP ; A trick: always taken800D: 60 11 DONE RTS ; Return

12800E: C8 E5 EC 13 MSG ASC "Hello World!" ; Generate ASCII character codes8011: EC EF A0 D78015: EF F2 EC E48019: A1801A: 8D 00 14 DB $8D,00 ; Define byte: 8D is return, 00 is end

--End assembly, 28 bytes, Errors: 0

Symbol table - alphabetical order:

COUT =$FDED DONE =$800D ? ENTRY =$8000 LOOP =$8002MSG =$800E

Symbol table - numerical order:

? ENTRY =$8000 LOOP =$8002 DONE =$800D MSG =$800ECOUT =$FDED

Bouncing Balls

doClear and display the Hires 1 screen

Draw the frameInitialize ball locations and velocitiesDraw each ball on the screendo

for each ball doErase the ballUpdate the ball’s horizontal position, possibly bouncingApply gravity to the ball’s vertical velocityUpdate the ball’s vertical position, bouncing at bottomDraw the ball

while no key pressedwhile the “R” key was pressed

Bouncing Balls

doClear and display the Hires 1 screenDraw the frame

Initialize ball locations and velocitiesDraw each ball on the screendo

Bouncing Balls

doClear and display the Hires 1 screenDraw the frameInitialize ball locations and velocities

Draw each ball on the screendo

Bouncing Balls

doClear and display the Hires 1 screenDraw the frameInitialize ball locations and velocitiesDraw each ball on the screen

dofor each ball do

Erase the ballUpdate the ball’s horizontal position, possibly bouncingApply gravity to the ball’s vertical velocityUpdate the ball’s vertical position, bouncing at bottomDraw the ball

Bouncing Balls

doClear and display the Hires 1 screenDraw the frameInitialize ball locations and velocitiesDraw each ball on the screendo

for each ball do

Erase the ballUpdate the ball’s horizontal position, possibly bouncingApply gravity to the ball’s vertical velocityUpdate the ball’s vertical position, bouncing at bottomDraw the ball

Bouncing Balls

for each ball doErase the ball

Update the ball’s horizontal position, possibly bouncingApply gravity to the ball’s vertical velocityUpdate the ball’s vertical position, bouncing at bottomDraw the ball

Bouncing Balls

for each ball doErase the ballUpdate the ball’s horizontal position, possibly bouncing

Apply gravity to the ball’s vertical velocityUpdate the ball’s vertical position, bouncing at bottomDraw the ball

Bouncing Balls

for each ball doErase the ballUpdate the ball’s horizontal position, possibly bouncingApply gravity to the ball’s vertical velocity

Update the ball’s vertical position, bouncing at bottomDraw the ball

Bouncing Balls

for each ball doErase the ballUpdate the ball’s horizontal position, possibly bouncingApply gravity to the ball’s vertical velocityUpdate the ball’s vertical position, bouncing at bottom

Draw the ballwhile no key pressed

while the “R” key was pressed

Bouncing Balls

while no key pressed

while the “R” key was pressed

Bouncing Balls

Apple II Text Mode: 40× 24, 7× 8 characters, 1 byte/char40

40 and 24 are not powers of 2

However,

8× 3 = 24

128 = 40× 3 + 8

8× 128 = 1024

Apple II Text Mode: 40× 24, 7× 8 characters, 1 byte/char

40 40 40 8128

$400 $428 $450 $478$480 $4A8 $4D0 $4F8$500 $528 $550 $578$580 $5A8 $5D0 $5F8$600 $628 $650 $678$680 $6A8 $6D0 $6F8$700 $728 $750 $778$780 $7A8 $7D0 $7F8

40 40 40 8128

8 128× 4 = 1024 = 1K

$400 $428 $450 $478$480 $4A8 $4D0 $4F8$500 $528 $550 $578$580 $5A8 $5D0 $5F8$600 $628 $650 $678$680 $6A8 $6D0 $6F8$700 $728 $750 $778$780 $7A8 $7D0 $7F8

40 40 40 8128

8 128× 4 = 1024 = 1K

$400 $428 $450 $478$480 $4A8 $4D0 $4F8$500 $528 $550 $578$580 $5A8 $5D0 $5F8$600 $628 $650 $678$680 $6A8 $6D0 $6F8$700 $728 $750 $778$780 $7A8 $7D0 $7F8

Apple II Lores: 40× 48, 16 colors, 4 bits/pixel, 1K/screen40

Apple II Hires: 140× 192, 6 colors, 8K/screen140

Apple II Hires Addresses: Even more bizarreY Hex Binary0 $2000 100000000000001 $2400 100100000000002 $2800 101000000000003 $2C00 101100000000004 $3000 110000000000005 $3400 110100000000006 $3800 111000000000007 $3C00 11110000000000

8 $2080 100000100000009 $2480 10010010000000

10 $2880 1010001000000011 $2C80 1011001000000012 $3080 1100001000000013 $3480 1101001000000014 $3880 1110001000000015 $3C80 1111001000000016 $2100 1000010000000017 $2500 1001010000000018 $2900 1010010000000019 $2D00 10110100000000

Apple II Hires Addresses: Even more bizarreY Hex Binary0 $2000 100000000000001 $2400 100100000000002 $2800 101000000000003 $2C00 101100000000004 $3000 110000000000005 $3400 110100000000006 $3800 111000000000007 $3C00 111100000000008 $2080 100000100000009 $2480 10010010000000

10 $2880 1010001000000011 $2C80 1011001000000012 $3080 1100001000000013 $3480 1101001000000014 $3880 1110001000000015 $3C80 11110010000000

16 $2100 1000010000000017 $2500 1001010000000018 $2900 1010010000000019 $2D00 10110100000000

10 $2880 1010001000000011 $2C80 1011001000000012 $3080 1100001000000013 $3480 1101001000000014 $3880 1110001000000015 $3C80 1111001000000016 $2100 1000010000000017 $2500 1001010000000018 $2900 1010010000000019 $2D00 10110100000000

0123456789101112131415

0 0 1 y2 y1 y0 y5 y4 y3 0 0 x4 x3 x2 x1 x0

Rows0–63 0 0 0 0

64–127 0 1 0 1

128–191 1 0 1 0

Woz used just a 4-bit adder

10 $2880 1010001000000011 $2C80 1011001000000012 $3080 1100001000000013 $3480 1101001000000014 $3880 1110001000000015 $3C80 1111001000000016 $2100 1000010000000017 $2500 1001010000000018 $2900 1010010000000019 $2D00 10110100000000

HPOSN STA HGRYSTX HGRXSTY HGRX+1PHAAND #%11000000STA GBASLLSR ALSR AORA GBASLSTA GBASLPLASTA GBASHASL AASL AASL AROL GBASHASL AROL GBASHASL AROR GBASLLDA GBASHAND #%00011111ORA HGRPAGESTA GBASHTXACPY #0BEQ HPOSN2LDY #35ADC #4

HPOSN1 INYHPOSN2 SBC #7

BCS HPOSN1STY HGRHORIZTAXLDA MSKTBL-249,XSTA HMASKTYALSR ALDA HGRCOLORSTA HCOLOR1BCS COLORSHIFTRTS

Solution: 384 Byte Address Lookup Tables

LKHI hex 2024282c3034383c2024282c3034383chex 2125292d3135393d2125292d3135393dhex 22262a2e32363a3e22262a2e32363a3ehex 23272b2f33373b3f23272b2f33373b3fhex 2024282c3034383c2024282c3034383c

LKLO hex 00000000000000008080808080808080hex 00000000000000008080808080808080hex 00000000000000008080808080808080hex 00000000000000008080808080808080hex 2828282828282828a8a8a8a8a8a8a8a8

Apple II Hires Colors: 7 pixels/2 bytes

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

palette pixels

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0

0 0 0 0 1 1 1 1 0 0 0 0 0 0 0 0

0 1 1 1 1 1 1 1 0 0 0 0 0 0 0 1

0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1

0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0

0 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1

0 1 1 0 1 0 1 0 0 0 1 0 1 0 1 1

1 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0

1 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1

1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 1

Preshifted Shapes: 7 versionsBALL0 db %00111100

db %01111111db %01111111db %01111111db %01111111db %01111111db %01111111db %00111100

db %01111000db %01111110db %01111110db %01111110db %01111110db %01111110db %01111110db %01111000

db %01110000db %01111100db %01111100db %01111100db %01111100db %01111100

BALL1 db %00000000db %00000001db %00000001db %00000001db %00000001db %00000001db %00000001db %00000000

db %00000000db %00000011db %00000011db %00000011db %00000011db %00000011db %00000011db %00000000

db %00000001db %00000111db %00000111db %00000111db %00000111db %00000111

Bouncing Balls: Clear the screen

hclear ldx #>HGR1SCRN ; $20, also the number of pages to clearstx GBASHlda #0 ; Clear to blacksta GBASLtay

hclr1 sta (GBASL),yinybne hclr1 ; Done with the page?inc GBASHdexbne hclr1 ; Done with all pages?bit HIRES ; Switch to hires modebit TXTCLRrts

hclr1 sta (GBASL),yinybne hclr1 ; Done with the page?

inc GBASHdexbne hclr1 ; Done with all pages?bit HIRES ; Switch to hires modebit TXTCLRrts

hclr1 sta (GBASL),yinybne hclr1 ; Done with the page?inc GBASHdexbne hclr1 ; Done with all pages?

bit HIRES ; Switch to hires modebit TXTCLRrts

hclr1 sta (GBASL),yinybne hclr1 ; Done with the page?inc GBASHdexbne hclr1 ; Done with all pages?bit HIRES ; Switch to hires modebit TXTCLRrts

Bouncing Balls: Horizontal line; Draw a horizontal line; A = color byte to repeat, e.g., $7F; Y = row (0-191) ($FF on exit);; Uses GBASL, GBASH

hline phalda LKLO,ysta GBASLlda LKHI,ysta GBASHldy #COLUMNS-1 ; Width of screen in bytespla

hl1 sta (GBASL),ydeybpl hl1rts

Bouncing Balls: Horizontal line; Draw a horizontal line; A = color byte to repeat, e.g., $7F; Y = row (0-191) ($FF on exit);; Uses GBASL, GBASHhline pha

lda LKLO,ysta GBASLlda LKHI,ysta GBASHldy #COLUMNS-1 ; Width of screen in bytespla

lda LKLO,ysta GBASLlda LKHI,ysta GBASH

ldy #COLUMNS-1 ; Width of screen in bytespla

Bouncing Balls: Vertical line; Draw a vertical line on all but the topmost and bottommost rows; A = byte to write in each position; Y = column;; Uses GBASL, GBASH, HCOLOR1vline sta HCOLOR1

ldx #190 ; Start at second-to-last rowvl1 lda LKLO,x ; Get the row address

sta GBASLlda LKHI,xsta GBASHlda HCOLOR1sta (GBASL),y ; Write the color bytedex ; Previous rowbne vl1rts

The Exclusive-OR Trick

A XOR B

B=0 B=1

A XOR B

B=0 B=1

A=1 1 0

A XOR B

Draw the Ball: XOR Two Bytes

(GBASL, GBASH) = row addressY = byte offset into the rowX = index into sprite tables

lda (GBASL),yeor BALL0,xsta (GBASL),yinylda (GBASL),yeor BALL1,xsta (GBASL),y

ldy HGRXlda (GBASL),yeor BALL0,xsta (GBASL),yinylda (GBASL),yeor BALL1,xsta (GBASL),y

Bouncing Balls: Draw a ball

xsplot ldy HGRY ; Get the row addresslda LKLO,ysta GBASLlda LKHI,ysta GBASHinysty HGRY

inxtxaand #7bne xsplot ; Stop at a multiple of 8 bytesrts

Bouncing Balls: Horizontal Position

BALLXH BALLXL0 0 c5 c4 c3 c2 c1 c0 0 0 s2 s1 s0 s−1 s−2 s−3

Byte column (0–39) = c5 c4 c3 c2 c1 c0

Bit/shift number (0.0–6.875) = s2 s1 s0 . s−1 s−2 s−3

Bouncing Balls: Horizontal Movement 1

lda BALLXL,xclcadc BALLDX,xbpl nounder

adc #56 ; Correct for underflow; carry is clearsta BALLXL,xdec BALLXH,xbne xdone ; Hit the left wall?beq bouncex ; Yes: bounce

Bouncing Balls: Horizontal Movement 2nounder sta BALLXL,x

secsbc #56bcc xdone ; No overflow?

sta BALLXL,xinc BALLXH,xldy BALLXH,xcpy #COLUMNS-2 ; Hit the right wall?bne xdone

bouncex seclda #0sbc BALLDX,xsta BALLDX,x

Bouncing Balls: Vertical Fall

inc BALLDYH,x ; Apply gravity

clc ; Update Ylda BALLYL,xadc BALLDYL,xsta BALLYL,xlda BALLYH,xadc BALLDYH,xsta BALLYH,x

cmp #>BOTTOM ; Did we hit the bottom?bcc nobounce

Bouncing Balls: Vertical Bouncesec ; We bounced: subtract Y from 2 * BOTTOMlda #<BOTTOM2sbc BALLYL,xsta BALLYL,xlda #>BOTTOM2sbc BALLYH,xsta BALLYH,x

sec ; and negate the vertical velocitylda #0sbc BALLDYL,xsta BALLDYL,xlda #0sbc BALLDYH,xsta BALLDYH,x

nobounce

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