Page 00

C64 Zero Page

Categories:

Address

Len

Name

Description

Default

0000

D6510

6510 On-Chip I/O DATA Direction Register

Bit 0	Direction of Bit 0 on I/O Port. Default = 1 Output
Bit 1	Direction of Bit 1 on I/O Port. Default = 1 Output
Bit 2	Direction of Bit 2 on I/O Port. Default = 1 Output
Bit 3	Direction of Bit 3 on I/O Port. Default = 1 Output
Bit 4	Direction of Bit 4 on I/O Port. Default = 0 Input
Bit 5	Direction of Bit 5 on I/O Port. Default = 1 Output
Bit 6	Direction of Bit 6 on I/O Port. Not used
Bit 7	Direction of Bit 7 on I/O Port. Not used

0xEF

0001

R6510

6510 On-Chip I/O Port

Bit 0	LORAM. Selects ROM/RAM at 0xA000. 1=Basic, 0=RAM
Bit 1	HIRAM. Selects ROM/RAM at 0xE000, 1=Kernal, 0=RAM
Bit 2	CHAREN. Selects Character ROM or I/O devices. 1=I/O, 0=ROM
Bit 3	Cassette Data Output line
Bit 4	Cassette switch sense. Reads 0=button pressed, 1=not pressed
Bit 5	Cassette Motor Control. 1=Motor on, 0=Motor off
Bit 6	Not used, not connected on 6510
Bit 7	Not used, not connected on 6510

0x37

0002

Unused

0003

ADRAY1

Vector to routine to convert Number from Floating Point to Signed Integer

0xB1AA

0005

ADRAY2

Vector to routine to convert Number from Integer to Floating Point

0xB391

0007

Used by BASIC and/or Kernal

000A

VERCK

Flag: LOAD or VERIFY

00	LOAD
01-FF	VERIFY

000B

Used by BASIC and/or Kernal

002B

TXTTAB

Pointer to start of BASIC program text

0x0801

002D

VARTAB

Pointer to start of BASIC Variable storage area

End of program +1

002F

ARYTAB

Pointer to start of array variable area

0031

STREND

End of Basic array storage (+1), Start of free ram

0033

FRETOP

Pointer to bottom of string text area

Grows downwards from end of BASIC area

0035

FRESPC

Temp pointer for strings

0037

BASMEMSIZ

Highest address used by basic

RS232 device can reduce this by 512 bytes for it's buffers

0x9FFF

0039

Used by BASIC

0090

Used by Kernal

00CC

BLNSW

Cursor Blink Enable

00	Cursor is on
01-FF	Cursor is off

00CD

Used by Kernal

00F7

RIBUF

RS232 Input Buffer Pointer

00xx for no buffer, allocate on input, any other value buffer exists

00F9

ROBUF

RS232 Output Buffer Pointer

00xx for no buffer, allocate on output, any other value buffer exists

00FB

FREKZP

4 free bytes of Zero Page for User Programs

Guaranteed BASIC will not use these 4 bytes

00FF

BASZPT

BASIC temp data area for floating point to ASCII conversion

Runs into start of page 1, free for use if conversions not needed

The C64 uses the 6510 Microprocessor, a variant of the 6502. The main difference is that this Microprocessor contains a built-in 8-bit I/O port, with 6 pins exposed on the 6510. This port is mapped into memory at addresses 0000 & 0001.

Last modified October 21, 2021: Change linkLabels to show what's in that page (4802cab)