JBit-QS Getting Started, System Overview, IO Basic Operations, Support Tables, 6502

JBit QS

Getting Started

If you are new to 6502 programming, read the online

tutorial and then run some programs step by step.

From the JBit menu, select Demos and then 6502.

Study all the programs (they are very short) in the

order they are presented. First select the option Info

to get some hints about the program, then select the

option Load&Debug to start the Monitor.

Monitor Status Bar: At the right there is the

indication of the current view (CPU or MEM). In

CPU view, at the left there is the PC of the next

instruction. In MEM view, at the left there is the

current cell address.

Monitor Keys: 2, 4, 6 and 8 are the cursor keys.

1 performs 1 step. 3 performs n steps (you can select

n by using the option StepSize; default is 10). 0

switches between CPU and MEM view. * continues

the program (you can then pause it by pressing a

soft key). # shows the video (you can then go back

to the monitor by pressing any key). In MEM view,

5 changes the value of a cell. The option Edit can be

used to change the registers of the CPU.

Editor Status Bar: At the right there is the indication

of the current major (NAV and EDT) and

minor modes. At the left there is the letter C if you

are on a code page or D if you are on a data page

followed by the current cell address.

Editor NAV Mode Keys: 2, 4, 6 and 8 are the

cursor keys. 5 switches to EDT mode. 1 goes to the

previous snap point, 3 to the next one. 7 sets the

mark and 0 swaps the cursor and the mark. 9 goes

to the address of the operand. * runs the program.

# switches between ASM and MEM minor modes.

Editor EDT Mode Keys: * moves to the next

cell. # switches between ASM and MEM minor

modes. In MEM minor mode, 0-9 are used to enter

the decimal value of the cell. In ASM minor mode,

2-9 are used to enter the letters of the mnemonic.

Editor Example: To enter LDA #12 press 5 to

enter EDT mode, then press # to enter ASM minor

mode, then press the sequence 5 JKL, 3 DEF and 2 ABC

for L-D-A, then select the instruction LDA #n from

the list, then press 1 and 2 to enter the operand and

finally select OK to confirm the instruction. Once a

few instructions have been entered you can then press

# in NAV mode to check if everything is correct.

On some phones, use @ for #.

System Overview

Neither interrupts nor BCD mode are supported.

SED, RTI or any invalid opcode cause the program to

abort. To end the program use the BRK (0) opcode.

By convention a program is a series of code pages

containing a single stream of valid instructions followed

by a series of data pages. Programs structured

in this way are said to be well-formed. JBit neither

needs nor enforces this convention, but some editing

operations (e.g. Resize) are only available on wellformed

programs.

The hexadecimal notation is cumbersome on mobile

phones; it is therefore common in JBit to use the

decimal notation. Absolute addresses are presented

in the non-standard page:offset notation.

The program is loaded and starts at the beginning

of page 3. Page 255 is reserved for future use. The

IO chip is mapped to page 2.

IO Basic Operations

IO registers are stated as decimal offsets relative to

page 2.

CONVIDEO 40 (40 bytes)

The console video memory is a 10x4 matrix of extended

Latin1 characters disposed in row-major order

(first the 10 characters of the first row from left to

right, then the second row, and so on).

CONROW0 40 CONROW1 50

CONROW2 60 CONROW3 70

The frame refresh is controlled by two registers:

FRMFPS 17 FRMDRAW 18

FRMFPS is the number of frames per second multiplied

by 4 (e.g. 40, the initial value, is 10 fps). Writing

0 to FRMDRAW causes the CPU to be suspended

until the current frame has been drawn.

The standard KeyPresses (the ones that can be

represented by ASCII codes; usually only 0-9, # and

*), are enqueued starting from KEYBUF; the rest of the

buffer is filled with 0s. Write 0 to KEYBUF to clear the

buffer or n to consume n KeyPresses. If the buffer is

full when a new key is pressed that KeyPress is lost.

KEYBUF 24 (8 bytes)

Copyright c

2007-2008 Emanuele Fornara JBit http://jbit.sourceforge.net/

JBit-QS Getting Started, System Overview, IO Basic Operations, Support Tables, 6502

Latin1

0 1 2 3 4 5 6 7 8 9

30 ! " # $ % & '

40 ( ) * + , - . / 0 1

50 2 3 4 5 6 7 8 9 : ;

60 < = > ? @ A B C D E

70 F G H I J K L M N O

80 P Q R S T U V W X Y

90 Z [ \ ] ^ _ ' a b c

100 d e f g h i j k l m

110 n o p q r s t u v w

120 x y z { | } ~

Line Art

Conversions

0 0000 0 0 8 1000 8 128

1 0001 1 16 9 1001 9 144

2 0010 2 32 10 1010 A 160

3 0011 3 48 11 1011 B 176

4 0100 4 64 12 1100 C 192

5 0101 5 80 13 1101 D 208

6 0110 6 96 14 1110 E 224

7 0111 7 112 15 1111 F 240

6502

Operations

BRK: BReaK; in JBit, used to halt the VM.

NOP: No OPeration.

LDA, LDX, LDY: LoaD Accumulator/X/Y.

STA, STX, STY: STore Accumulator/X/Y.

INX, INY, INC: INCrement X/Y/memory.

DEX, DEY, DEC: DECrement X/Y/memory.

TAX, TAY, TXA, TYA, TSX, TXS: Transfer register.

CMP, CPX, CPY: CoMPare with accumulator/X/Y.

JMP: JuMP.

JSR: Jump to SubRoutine.

RTS: ReTurn from Subroutine.

CLC, CLV: CLear Carry/oVerflow.

SEC: SEt Carry.

BEQ, BNE: Branch if EQual/Not Equal (z flag).

BMI, BPL: Branch if MInus/PLus (n flag).

BCC, BCS: Branch if Carry Clear/Set.

BVC, BVS: Branch if oVerflow Clear/Set.

ADC: ADd (to/into accumulator) using Carry.

SBC: SuBtract (from/into accumulator) using Carry.

AND: bitwise AND (with accumulator).

ORA: bitwise inclusive OR (with Accumulator).

EOR: bitwise Exclusive OR (with accumulator).

BIT: test BITs: #6 to v flag and #7 to n flag.

ASL, LSR: Arithmetic/Logical Shift Left/Right.

ROL, ROR: ROtate Left/Right.

PHA, PLA: PusH/PuLl Accumulator.

PHP, PLP: PusH/PuLl Processor status.

Operands

#n: Constant n.

n: Cell 0:n.

n:n: Cell n:n.

n:n,X: Cell X cells after n:n.

n:n,Y: Cell Y cells after n:n.

r: Next cell + relative offset r, shown as n:n.

n,X: Cell 0:(n+X modulo 256).

n,Y: Cell 0:(n+Y modulo 256).

(n:n): Cell pointed by n:n.

(n,X): Cell pointed by 0:(n+X modulo 256).

(n),Y: Cell Y cells after cell pointed by 0:n.

Examples

TAX: Transfer (i.e. copy) Accumulator into X.

LDA #2: LoaD Accumulator with constant 2.

LDX 2: LoaD X with content of cell 0:2.

STX 2:18: STore (i.e copy) X into cell 2:18.

BCC 6: If C=0, skip (i.e. jump forward) 6 bytes.

BCC 240: If C=0, jump back 16 (i.e. 256-240) bytes.

Copyright c

2007-2008 Emanuele Fornara JBit http://jbit.sourceforge.net/