Zeropage

The first 256 Bytes of the C64 memory map (0-255 or $0000-$00FF) are called zeropage (Page 0). Besides faster fetching and storing from/to these addresses, the 65xx CPUs offer the indirect-indexed addressing mode which only work on zeropage addresses.

The two very first addresses in both zeropage (and the entire address space) of the C64, are "hardwired" in the CPU to internal registers for use with the 6510's on-chip port. Reading and writing to these registers does not affect the attached RAM chips, and there is no direct way to access addresses 0 and 1 in RAM rather than the internal port registers (but some cunning hardware trickery with the VIC-chip will allow you to do so). This leaves 254 "normal", usable zeropage addresses, and thus potentially 254/2=127 index registers for use with indirect-indexed addressing.

This is also part of the reason why there are so few internal registers in 65xx CPUs compared to other CPUs of the day (1970s design): Why waste precious space on the CPU chip itself, when external RAM (including the zeropage area) runs at the same speed as the CPU itself, and thus provides a whopping 127 16-bit index registers...!

Those zeropage addresses are useful and important in programming the C64 – so useful that the internal KERNAL and BASIC system ROMs make use of most of them, leaving only a handful that are completely "safe" for machine language programmers to use for their own purposes, without risking a system crash.

Addresses

Hex Address	Dec Address	Type	Purpose
$0000	0	Register	6510 CPU's data direction I/O port register; 0 = input, 1 = output
			Bit 0 - Direction of I/O on port at bit 0 of address $0001; default = 1
			Bit 1 - Direction of I/O on port at bit 1 of address $0001; default = 1
			Bit 2 - Direction of I/O on port at bit 2 of address $0001; default = 1
			Bit 3 - Direction of I/O on port at bit 3 of address $0001; default = 1
			Bit 4 - Direction of I/O on port at bit 4 of address $0001; default = 0
			Bit 5 - Direction of I/O on port at bit 5 of address $0001; default = 1
			Bit 6 - Direction of I/O on port at bit 6 of address $0001; unused, as bit 6 of address $0001 is undefined
			Bit 7 - Direction of I/O on port at bit 7 of address $0001; unused, as bit 7 of address $0001 is undefined
$0001	1	Register	6510 CPU's on-chip port register
			Bit 0 - LORAM: Configures RAM or ROM at $A000-$BFFF (see bankswitching)
			Bit 1 - HIRAM: Configures RAM or ROM at $E000-$FFFF (see bankswitching)
			Bit 2 - CHAREN: Configures I/O or ROM at $D000-$DFFF (see bankswitching)
			Bit 3 - Cassette Data Output Line (Datasette)
			Bit 4 - Cassette Switch Sense; 1 = Switch Closed
			Bit 5 - Cassette Motor Control; 0 = On, 1 = Off
			Bit 6 - Undefined
			Bit 7 - Undefined
$0002	2		Unused
$0003	3	Pointer	Low Byte pointing to address 45482/$B1AA; a ROM routine for converting floating point numbers to integers
$0004	4	Pointer	High byte pointing to address 45482/$B1AA; a ROM routine for converting floating point numbers to integers
$0005	5	Pointer	Low Byte pointing to 45969/$B391; a ROM routine for converting integer numbers to floating point format
$0006	6	Pointer	High byte pointing to 45969/$B391; a ROM routine for converting integer numbers to floating point format
$0007	7	Flag	Search character/temporary integer during INT, OR and AND
$0008	8	Flag	Scan for quote character at end of string during tokenization of BASIC commands.
$0009	9	Value	Cursor column position after last invocation of TAB or SPC
$000A	10	Flag	Sets condition for BASIC Interpreter function; 0 = LOAD, 1 = VERIFY
$000B	11	Pointer	Input buffer pointer/number of subscripts for DIM, line length for tokenization
$000C	12	Flag	Default size of array for DIM
$000D	13	Flag	Variable type; 0 = numeric, 255 = character string
$000E	14	Flag	Numeric variable type flag; 0 = floating-point, 128 = integer
$000F	15	Flag	For LIST, Garbage Collection or tokenization
$0010	16	Flag	Distinguishes between user function call or array variable
$0011	17	Flag	Designates data entry method; 0 = INPUT, 64 = GET or 152 = READ
$0012	18	Flag	Tracks sign of trigonometric function calls (255 in quadrant 2/3 for SIN or TAN or quadrant 1/2 for COS, 0 otherwise), or tracks comparison operator (1 = >, 2 = equals, 4 = <, is bit-field combination)
$0013	19	Flag	Current input filenumber: 0 = default input (Keyboard), > 0 other open file
$0014	20	Pointer	Low Byte target line number for ON, GOTO, GOSUB and LIST, and address for PEEK, POKE and SYS
$0015	21	Pointer	High byte target line number for ON, GOTO, GOSUB and LIST, and address for PEEK, POKE and SYS
$0016	22	Pointer	Next available element on the temporary string descriptor stack in address range from 25 to 33 ($0019-$0021)
$0017	23	Pointer	Low Byte of last used temporary string address or string stack pointer; value = 3 less than the value at address 22 ($0016)
$0018	24	Pointer	High byte of last used temporary string address or string stack pointer; value = 0
$0019-$0021	25-33		Temporary string stack: holds 3 string descriptors each consisting of length and start address of string
$0022-$0025	34-37	Pointer	Utility pointer area for the BASIC interpreter
$0026-$002A	38-42		Floating point result of multiplication or division
$002B	43	Pointer	Low Byte TXTTAB, start of BASIC program text, default: 2049 / $0801
$002C	44	Pointer	High byte TXTTAB, start of BASIC program text, default: 2049 / $0801
$002D	45	Pointer	Low Byte VARTAB, end of BASIC program text+1 / start of numeric variables
$002E	46	Pointer	High byte VARTAB, end of BASIC program text+1 / start of numeric variables
$002F	47	Pointer	Low Byte ARYTAB, end of numeric Variables+1 / start of array variables
$0030	48	Pointer	High byte ARYTAB, end of numeric Variables+1 / start of array variables
$0031	49	Pointer	Low Byte STREND, end of array variables+1 / lowest address for string stack
$0032	50	Pointer	High byte STREND, end of array variables+1 / lowest address for string stack
$0033	51	Pointer	Low Byte FRETOP, top of string stack
$0034	52	Pointer	High byte FRETOP, top of string stack
$0035	53	Pointer	Low Byte FRESPC, utility pointer for strings
$0036	54	Pointer	High byte FRESPC, utility pointer for strings
$0037	55	Pointer	Low Byte MEMSIZ, highest BASIC RAM address / bottom of string stack
$0038	56	Pointer	High byte MEMSIZ, highest BASIC RAM address / bottom of string stack
$0039	57	Pointer	Low Byte current BASIC line number
$003A	58	Pointer	High byte current BASIC line number
$003B	59	Pointer	Low Byte previous BASIC line number for CONT after STOP
$003C	60	Pointer	High byte previous BASIC line number for CONT after STOP
$003D	61	Pointer	Low Byte next BASIC statement for CONT
$003E	62		High byte next BASIC statement for CONT
$003F	63		Low Byte current DATA line
$0040	64		High byte current DATA line
$0041-$0042	65-66		next DATA item for READ
$0043	67		temporary storage for INPUT
$0044	68		temporary storage for INPUT
$0045	69		Name of variable to look up in variable table (VARNAM).
$0046	70		Name of variable to look up in variable table (VARNAM).
$0047	71		VARNAM of current variable if integer; to descriptor if string
$0048	72		VARNAM of current variable if integer; to descriptor if string
$0049	73		Index variable of FOR ... NEXT loop
$004A	74		Index variable of FOR ... NEXT loop
$004B	75		Temporary storage for mathematical operations or TXTPTR during READ/GET/INPUT
$004C	76		Temporary storage for mathematical operations or TXTPTR during READ/GET/INPUT
$004D	77		Mask used during evaluation (FRMEVL) of <, >, =
$004E-$004F	78-79		Temporary storage for FN or floating point value during FLPT
$0050-$0052	80-82		Strings
$0053	83		Length of string variable during Garbage Collection
$0054	84	Constant	Processor opcode "JMP absolute"; value = 76
$0055-$0056	85-86		Pointer during function evaluation
$0057-$005B	87-91		Register for TAN (floating point accumulator (FAC) #3)
$005C-$0060	92-96		Register for TAN (FAC #4)
$0061	97		Floating point accumulator (FAC) #1 exponent
$0062	98		Floating point accumulator (FAC) #1 mantissa
$0063	99		Floating point accumulator (FAC) #1 mantissa
$0064	100		Floating point accumulator (FAC) #1 mantissa
$0065	101		Floating point accumulator (FAC) #1 mantissa
$0066	102		Floating point accumulator (FAC) #1 sign
$0067	103		Number of terms in series evaluation
$0068	104		FAC #1: bit overflow area during normalization
$0069	105		Floating point accumulator (FAC) #2 exponent
$006A	106		Floating point accumulator (FAC) #2 mantissa
$006B	107		Floating point accumulator (FAC) #2 mantissa
$006C	108		Floating point accumulator (FAC) #2 mantissa
$006D	109		Floating point accumulator (FAC) #2 mantissa
$006E	110		Floating point accumulator (FAC) #2 sign
$006F	111	Flag	Result of signed comparison between FAC #1 and FAC #2: 0 = equal signs, 255 = different signs
$0070	112		FAC #2: low byte of FAC #1 mantissa during rounding if mantissa bigger longer than four bytes
$0071-$0072	113-114	Pointer	To temporary table during series evaluation
$0073-$008A	115-138		CHRGET routine: fetches next character of BASIC program text
$008B-$008F	139-143		Seed stored by last invocation of RND
$0090	144		KERNAL I/O STATUS indicates end of file if bit 6 is set
$0091	145	Flag	127 = stop , 223 = C= , 239 = SPACE , 251 = CTRL , 255 = no key pressed
$0092	146		Constant for timing of cassette reads
$0093	147	Flag	KERNAL LOAD routine: 0 = LOAD, 1 = VERIFY
$0094	148	Flag	Character waiting in serial bus output register
$0095	149		Serial bus output register
$0096	150		Cassette block synchronisation number
$0097	151		Temporary storage for X register
$0098	152		Number of open I/O file; pointer to top of file table (see addresses 601-631)
$0099	153		Current input device: defaults to 0 = keyboard
$009A	154		Current output device for CMD: defaults to 3 = screen
$009B	155		Cassette parity byte
$009C	156	Flag	Tape byte received
$009D	157	Flag	KERNAL message display control: bit 6 = error messages, bit 7 = control message
$009E	158		Cassette read pass 1 error log
$009F	159		Cassette read pass 2 error log
$00A0-$00A2	160-162		Software jiffy clock, updated by KERNAL IRQ every 1/60 second
$00A3-$00A4	163-164		Bit counter for serial bus / cassette I/O (EOI)
$00A5	165		Cassette synchronization byte counter
$00A6	166		Number of characters in cassette I/O buffer
$00A7	167		Temporary RS-232 / cassette read data register
$00A8	168	Counter	RS-232 input bits received / flag: cassette read error
$00A9	169	Flag	RS-232 start bit 0 = received, 144 = not received
$00AA	170		RS-232 input byte buffer / flag: cassette read character sync/data
$00AB	171		RS-232 input parity/cassette leader counter
$00AC	172	Pointer	Starting address of a load / screen scrolling temporary storage
$00AD	173	Pointer	Starting address of a load / screen scrolling temporary storage
$00AE	174	Pointer	End address of LOAD/VERIFY/SAVE
$00AF	175	Pointer	End address of LOAD/VERIFY/SAVE
$00B0	176	Pointer	Constant for timing of cassette reads (default: 146)
$00B1	177	Pointer	Constant for timing of cassette reads (default: 146)
$00B2	178	Pointer	Low byte Start of cassette buffer
$00B3	179	Pointer	High byte Start of cassette buffer
$00B4	180		RS-232 output bit counter / cassette ready to read next byte
$00B5	181		Next output bit / cassette read block byte counter
$00B6	182		Output buffer
$00B7	183		Length of current file name
$00B8	184		Current logical file number
$00B9	185		Current secondary address
$00BA	186		Current device number
$00BB	187	Pointer	Current file name
$00BC	188	Pointer	Current file name
$00BD	189		RS-232 output parity / temporary cassette read/write register
$00BE	190		cassette read/write duplicate block counter
$00BF	191		Cassette read byte register
$00C0	192	Flag	Cassette motor 0 = off, 1 = on
$00C1	193		Start address for LOAD / SAVE
$00C2	194		Start address for LOAD / SAVE
$00C3	195		End address for LOAD / SAVE
$00C4	196		End address for LOAD / SAVE
$00C5	197		Matrix coordinate of last pressed key, 64 = none
$00C6	198		Number of characters in keyboard buffer
$00C7	199	Flag	Print reverse characters, default 0 = normal
$00C8	200	Pointer	Last column of current line during INPUT
$00C9	201	Pointer	X coordinate of cursor during INPUT
$00CA	202		Y coordinate of cursor during INPUT
$00CB	203		Index to keyboard decoding table for currently pressed key, 64 = no key was depressed
$00CC	204	Flag	Flash cursor 0 = yes, otherwise no
$00CD	205		Counter for cursor flashing, default 20, decreased every jiffy
$00CE	206		Character at cursor position
$00CF	207	Flag	Cursor flash phase 0 = visible, >0 invisible
$00D0	208	Flag	Input from 0 = keyboard or 3 = screen
$00D1	209	Pointer	Low byte memory address of current screen line
$00D2	210	Pointer	High byte memory address of current screen line
$00D3	211		Column of cursor in current logical line, used by SPC(
$00D4	212	Flag	Quote mode, >0 = editor is in quote mode, i.e. control characters are printed as reverse graphic characters
$00D5	213		Maximum column index of logical screen line (39 or 79)
$00D6	214		Current physical line number of cursor
$00D7	215		ASCII value of last printed character
$00D8	216	Flag	Insert mode, if > 0 puts editor in quote mode (see address 212 ($00D4))
$00D9-$00F2	217-242		Screen line link table / temporary storage for editor; 26 bytes, one byte for each screen line: bit 0…6 containing to the page based on the start of screen RAM page in $288/648; bit 7 reset indicates that the line together with the previous line forms a logical line longer than 40 characters. $F2/242 is for a fictive overflow line while scrolling.
$00F3	243	Pointer	Low byte first address in Color RAM of current screen line
$00F4	244	Pointer	High byte first address in Color RAM of current screen line
$00F5	245	Pointer	Low byte keyboard decoding table
$00F6	246	Pointer	High byte keyboard decoding table
$00F7	247	Pointer	Low byte RS-232 input buffer
$00F8	248	Pointer	High byte RS-232 input buffer
$00F9	249	Pointer	Low byte RS-232 output buffer
$00FA	250	Pointer	High byte RS-232 output buffer
$00FB	251		Unused
$00FC	252		Unused
$00FD	253		Unused
$00FE	254		Unused
$00FF	255		Temporary register for BASIC floating point to ASCII conversion