Commodore VIC-20 Memory Map

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Here is the complete VIC-20 memory map, to be used as reference. At the end, a few tips. Feel free to share!

Below you will find a first complete map, then in an additional section we review in detail the most important pointers used by BASIC. Finally, a scheme of the memory in the different VIC configurations, and a few tips.

  • If you are using BASIC commands POKE and PEEK, then use addresses in the second columns (Decimal values).
  • If you are using VICMON or other assembly utilities, then the Hexadecimal addresses (first column) are probably most useful to you.
  • In the VIC docs like this, a number preceded by the dollar sign $ such as $A000 denotes a hexadecimal address. If you need it, here is an hexadecimal-decimal converter
  • Addresses marked with an asterisk *: Useful memory locations

 

HEX address
(VICMON)		 DEC address
(POKE/PEEK)		 Purpose
0000 0 Jump for USR
0001-0002 1-2 Vector for USR
0003-0004 3-4 Float-Fixed vector
0005-0006 5-6 Fixed-Float vector
0007 7 Search character
0008 8 Scan-quotes flag
0009 9 TAB column save
000A 10 0=LOAD, 1=VERIFY
000B 11 Input buffer pointer/# subscript
000C 12 Default DIM flag
000D 13 Type: FF=string, 00=numeric
000E 14 Type: 80=integer, 00=floating point
000F 15 DATA scan/LlST quote/memory flag
0010 16 Subscript/FNx flag
0011 17 0 = INPUT;$40 = GET;$98 = READ
0012 18 ATN sign/Comparison eval flag
0013 19 Current l/O prompt flag
0014-0015 20-21 Integer value
Where BASIC stores integer variables
used in calculations. The fixed-float and
float-fixed routines (vectors at 1-2 and
3-4) use the value in this area.
0016 22 Pointer: temporary string stack
0017-0018 23-24 Last temp string vector
0019-0021 25-33 Stack for temporary strings
0022-0025 34-37 Utility pointer area
0026-002A 38-42 Product area for multiplication
*002B-002C 43-44 Pointer: Start of Basic
Location 43 contains the low
byte, and location 44 has the high byte.
To compute the start of BASIC in
decimal, use the formula: PEEK(43) +
256 * PEEK(44)
*002D-002E 45-46 Pointer: Start of numeric Variables
which is usually immediately after the
end of the BASIC program.
*002F-0030 47-48 Pointer: Start of Arrays
*0031-0032 49-50 Pointer: End of Arrays
*0033-0034 51-52 Pointer: String storage (moving down)
Bottom of string storage, moving from
the top of available memory down to the
top of arrays.
0035-0036 53-54 Utility string pointer
*0037-0038 55-56 Pointer: Limit of memory
The top of free RAM. By lowering this
value, some RAM can be “protected”
against BASIC putting values here.
0039-003A 57-58 Current Basic line number
003B-003C 59-60 Previous Basic line number
003D-003E 61-62 Pointer: Basic statement for CONT
003F-0040 63-64 Current DATA line number
0041-0042 65-66 Current DATA address
*0043-0044 67-68 Jump vector for INPUT statement.
0045-0046 69-70 Current variable name
0047-0048 71-72 Current variable address
0049-004A 73-74 Variable pointer for FOR/NEXT
004B-004C 75-76 Y-save; op-save; Basic pointer save
004D 77 Comparison symbol accumulator
004E-0053 78-83 Misc work area, pointers, etc
0054-0056 84-86 Jump vector for functions
0057-0060 87-96 Misc numeric work area
*0061 97 Floating point accumulator #1 for cal-
culations: Exponent
*0062-0065 98-101 Floating point accumulator #1 for cal-
culations: Mantissa
*0066 102 Floating point accumulator #1 for cal-
culations: Sign
0067 103 Series evaluation constant pointer
0068 104 Accum#1 hi-order (overflow)
*0069-006E 105-110 Floating point accumulator #2: Exponent, etc.
006F 111 Sign comparison, Acc#1 vs #2
0070 112 Accum#1 lo-order (rounding)
0071-0072 113-114 Cassette buffer length/Series pointer
*0073-008A 115-138 CHRGET subroutine (get BASIC char)
This machine code routine gets the next BASIC
character from machine language.
007A-007B 122-123 Basic pointer (within subroutine)
008B-008F 139-143 RND seed value
*0090 144 Status word ST
0091 145 Keyswitch PIA: STOP and RVS flags
0092 146 Timing constant for tape
0093 147 Load=0, Verify=1
0094 148 Serial output: deferred char flag
0095 149 Serial deferred character
0096 150 Tape EOT received
0097 151 Register save
*0098 152 Number of open files.
*0099 153 Device number for input, normally 0
(keyboard).
*009A 154 Output (CMD) device, normally 3 (screen).
009B 155 Tape character parity
009C 156 Byte-received flag
009D 157 Direct=$80/RUN=0 output control
009E 158 Tape Pass 1 error log/char buffer
009F 159 Tape Pass 2 error log corrected
*00A0-00A2 160-162 3 byte jiffy clock. The Tl and Tl$ variables are translations of these locations.
00A3 163 Serial bit count/EOI flag
00A4 164 Cycle count
00A5 165 Countdown, tape write/bit count
00A6 166 Pointer: tape buffer
00A7 167 Tape Write ldr count/Read pass/inbit
00A8 168 Tape Write new byte/Read error/inbit
cnt
00A9 169 Write start bit/Read bit err/stbit
00AA 170 Tape Scan;Cnt;Ld;End/byte assy
00AB 171 Write lead length/Rd checksum/parity
00AC-00AD 172-173 Pointer: tape buffer, scrolling
00AE-00AF 174-175 Tape end addresses/End of program
00B0-00B1 176-177 Tape timing constants
*00B2-00B3 178-179 Pointer: start of tape buffer.
Can be used as an indirect zero-page jump
to a routine in the buffer.
00B4 180 Tape timer (1 =enable); bit cnt
00B5 181 Tape EOT/RS-232 next bit to send
00B6 182 Read character error/outbyte buffer
*00B7 183 Number of characters in filename.
*00B8 184 Current logical file
*00B9 185 Current secondary address. Which secondary address is currently
being used.
*00BA 186 Current device number being accessed.
*00BB-00BC 187-188 Points to location of filename in memory
00BD 189 Write shift word/Read input char
00BE 190 # blocks remaining to Write/Read
00BF 191 Serial word buffer
00C0 192 Tape motor interlock
00C1-00C2 193-194 I/O start addresses
00C3-00C4 195-196 KERNAL setup pointer
*00C5 197 Current key being held down. There will be a 64 here if nothing is held down. If more than 1 key is down, the key with the highest number on the chart is what shows up here.

# key # key # key # key
0 1 16 none 32 space 48 Q
1 3 17 A 33 Z 49 E
2 5 18 D 34 C 50 T
3 7 19 G 35 B 51 U
4 9 20 J 36 M 52 O
5 + 21 L 37 . 53 @
6 � 22 ; 38 none 54 ^ (up arrow)
7 DEL 23 cursor left/right 39 f1 55 f5
8 <- 24 STOP 40 none 56 2
9 W 25 none 41 S 57 4
10 R 26 X 42 F 58 6
11 Y 27 V 43 H 59 8
12 I 28 N 44 K 60 0
13 P 29 , 45 : 61 –
14 * 30 / 46 = 62 HOME
15 RETURN 31 cursor up/down 47 f3 63 f7
*00C6 198 Number chars in keyboard buffer
*00C7 199 Flag for reverse on/off. A 1 here is on, a
0 is off.
00C8 200 Pointer: End-of-line for input
00C9-00CA 201-202 Input cursor log (row, column)
*00CB 203 Same as 197. Which key pressed: 64 if no key
00CC 204 cursor enable (0=flash cursor)
00CD 205 Cursor timing countdown
00CE 206 Character under cursor
00CF 207 Cursor in blink phase
00D0 208 Input from screen/from keyboard
*00D1-00D2 209-210 Pointer to screen line. Address of start of line where cursor is.
*00D3 211 Position of cursor on above line
00D4 212 0=direct cursor, else programmed
*00D5 213 Current screen line length–either 21,
43, 65, or 87.
*00D6 214 Row where cursor lives. To change
the cursor position, locations 209, 210,
211, and 214 must be changed.
00D7 215 Last inkey/checksum/buffer
*00D8 216 Number of spaces left in INSERT mode.
POKEing this to a zero will turn off insert
mode.
*00D9-00F0 217-240 Screen line link table. A 158 means that
the line is finished at the end of that line,
and a 30 means that the line continues
on the next line.
00F1 241 Dummy screen link
00F2 242 Screen row marker
*00F3-00F4 243-244 Pointer: the current space in color memory.
00F5-00F6 245-246 Keyboard pointer
00F7-00F8 247-248 RS-232 Rcv pointer
00F9-00FA 249-250 RS-232 Tx pointer
*00FB-00FE 251-254 Available locations in zero page.
00FF 255 Basic storage
0100-010A 256-266 Floating to ASCII work area
0100-013E 256-318 Tape error log
0100-01FF 256-511 CPU stack area
*0200-0258 512-600 Basic input buffer (where the characters being INPUT will go)
*0259-0262 601-610 Logic 1 file table for OPEN files.
*0263-026C 611-620 Device # table for OPEN files.
*026D-0276 621-630 Secondary Address table
*0277-0280 631-640 Keyboard buffer. If characters are POKEd in
here and location 198 (# of characters in
buffer) is changed, it will be as if the
characters were typed from the keyboard.
*0281-0282 641-642 Pointer: Start of memory
*0283-0284 643-644 Pointer: Top of memory
0285 645 Serial bus timeout flag
*0286 646 Current color code. This holds the color
number that goes into color memory
during PRINT operations.
0287 647 Color under cursor
*0288 648 Screen memory page. If you want the
operating system to know where screen
memory is, this must be changed as well
as the VIC chip.
*0289 649 Max size of keyboard buffer. If this
is set greater than 10, vital pointers will
be destroyed.
*028A 650 Keyboard repeat flag. If this is a 0, only
cursor controls repeat; if 128, all keys
repeat.
*028B 651 Repeat speed counter. This determines how long the VIC waits
before repeating key.
028C 652 Repeat delay counter
*028D 653 Keyboard Shift/Control/Commodore flag. The SHIFT
sets the 1 bit, Commodore sets the 2 bit, and
the CTRL sets the 4 bit.
028E 654 Last keyboard shift pattern
028F-0290 655-656 Pointer: decode logic
*0291 657 Shift mode switch (0 = enabled, 128 = locked). Setting this location to 128 will disable
switching case, and a 0 here enables
the ability to switch.
0292 658 Autoscrolldownflag (0=on, <>0=off)
0293 659 RS-232 control register
0294 660 RS-232 command register
0295-0296 661-662 Nonstandard (Bit time/2-100)
0297 663 RS-232 status register
0298 664 Number of bits to send
0299-029A 665-666 Baud rate (full) bit time
029B 667 RS-232 receive pointer
029C 668 RS-232 input pointer
029D 669 RS-232 transmit pointer
029E 670 RS-232 output pointer
029F-02A0 671-672 Holds IRQ during tape operations
02A1-02FF 673-767 Program indirects
*0300-0301 768-769 This is the jump vector for errors. By
changing this vector, a routine can
intercept any error condition.
0302-0303 770-771 Basic warm start link
0304-0305 772-773 Crunch Basic tokens link
0306-0307 774-775 Print tokens link
0308-0309 776-777 Start new Basic code link
030A-030B 778-779 Get arithmetic element link
030C 780 Storage for 6502 .A register
030D 781 Storage for 6502 .X register
030E 782 Storage for 6502 .Y register
030F 783 Storage for 6502 .P register
0310-0313 784-787 ??
0314-0315 788-789 Hardware (IRQ) interrupt vector [EABF]
0316-0317 790-791 Break interrupt vector [FED2]
0318-0319 792-793 NMI interrupt vector [FEAD]
031A-031B 794-795 OPEN vector [F40A]
031C-031D 796-797 CLOSE vector [F34A]
031E-031F 798-799 Set-input vector [F2C7]
0320-0321 800-801 Set-output vector [F309]
0322-0323 802-803 Restore l/O vector [F3F3]
0324-0325 804-805 INPUT vector [F20E]
0326-0327 806-807 Output vector [F27A]
0328-0329 808-809 Test-STOP vector [F770]
032A-032B 810-811 GET vector [F1F5]
032C-032D 812-813 Abort l/O vector [F3EF]
032E-032F 814-815 User vector (default BRK) [FED2]
0330-0331 816-817 Link to load RAM [F549]
0332-0333 818-819 Link to save RAM [F685]
0334-033B 820-827 ??
033C-03FB 828-1019 Cassette buffer
This is where data files are held before they are INPUT. When not using files, this is available for POKEing or machine language programs.
0400-0FFF 1024-4095 3K expansion RAM area
for 3k-expanded VIC: User Basic area starts here
1000-1DFF 4096-7679 for Unexpanded VIC: User Basic area
for 8k+ VIC: Screen Memory until $11FF, then User Basic area from $1200 upwards
1000-11FF 4096-4607 for 8k+ VIC: Screen Memory
1200- 4608- for 8k+ VIC: User Basic area
1E00-1FFF 7680-8191 for Unexpanded VIC: Screen memory
2000-3FFF 8192-16383 BLK 1 – 8K expansion RAM/ROM block 1
4000-5FFF 16384-24575 BLK 2 – 8K expansion RAM/ROM block 2
6000-7FFF 24576-32767 BLK 3 – 8K expansion RAM/ROM block 3
8000-8FFF 32768-36863 4K Character generator ROM
8000-83FF 32768-33791 Upper case and graphics
8400-87FF 33792-33815 Reversed upper case and graphics
8800-8BFF 33816-35839 Upper and lower case
8C00-8FFF 35840-36863 Reversed upper and lower case
9000-93FF 36864-37887 I/O BLOCK 0 (VIA)
9000-900F 36864-36879 Address of VIC chip registers
9000 36864 bits 0-6 horizontal centering
bit 7 sets interlace scan
9001 36865 vertical centering
9002 36866 bits 0-6 set # of columns
bit 7 is part of video matrix address
9003 36867 bits 1-6 set # of rows
bit 0 sets 8×8 or 16×8 chars
9004 36868 TV raster beam line
9005 36869 bits 0-3 start of character memory
(default = 0)

Bits 4-7 is rest of video address

(default = F)

BITS 3,2,1,0 CM starting address
    HEX DEC
0000 ROM 8000 32768
0001   8400 33792
0010   8800 34816
0011   8C00 35840
1000 RAM 0000 0000
1001   xxxx  
1010   xxxx  
1011   xxxx  
1100   1000 4096
1101   1400 5120
1110   1800 6144
1111   1C00 7168
9006 36870 horizontal position of light pen
9007 36871 vertical position of light pen
9008 36872 Digitized value of paddle X
9009 36873 Digitized value of paddle Y
900A 36874 Frequency for oscillator 1 (low)
(on: 128-255)
900B 36875 Frequency for oscillator 2 (medium)
(on: 128-255)
900C 36876 Frequency for oscillator 3 (high)
(on: 128-255)
900D 36877 Frequency of noise source
900E 36878 bit 0-3: volume of all sound
bits 4-7: auxiliary color information
900F 36879 Screen and border color register
bits 4-7 select background color
bits 0-2 select border color
bit 3 selects inverted or normal mode
9110-911F 37136-37151 6522 VIA#1
9110 37136 Port B output register (user port and RS-232 lines)

PIN 6522 DESCRIPTION EIA ABV
C PB0 Received data (BB) Sin
D PB1 Request to Send (CA) RTS
E PB2 Data terminal ready (CD) DTR
F PB3 Ring indicator (CE) RI
H PB4 Received line signal (CF) DCD
J PB5 Unassigned () XXX
K PB6 Clear to send (CB) CTS
L PB7 Data set ready (CC) DSR
B CB1 Interrupt for Sin (BB) Sin
M CB2 Transmitted data (BA) Sout
A GND Protective ground (M) GND
N GND Signal ground (AB) GND
9111 37137 Port A output register
(PA0) Bit 0=Serial CLK IN
(PA1) Bit 1=Serial DATA IN
(PA2) Bit 2=Joy 0
(PA3) Bit 3=Joy 1
(PA4) Bit 4=Joy 2
(PA5) Bit 5 = Lightpen/Fire button
(PA6) Bit 6=Cassette switch sense
(PA7) Bit 7=Serial ATN out
9112 37138 Data direction register B
9113 37139 Data direction register A
9114 37140 Timer 1 low byte
9115 37141 Timer 1 high byte & counter
9116 37142 Timer 1 low byte
9117 37143 Timer 1 high byte
9118 37144 Timer 2 low byte
9119 37145 Timer 2 high byte
911A 37146 Shift register
911B 37147 Auxiliary control register
911C 37148 Peripheral control register
(CA1, CA2, CB1, CB2)
CA1 = restore key (Bit 0)
CA2 = cassette motor control (Bits 1-3)
CB1 = interrupt signal for received
RS-232 data (Bit 4)
CB2=transmitted RS-232 data (Bits
5-7)
911D 37149 Interrupt flag register
911E 37150 Interrupt enable register
911F 37151 Port A (Sense cassette switch)
9120-912F 37152-37167 6522 VIA#2
9120 37152 Port B output register
keyboard column scan
(PB3) Bit 3 =cassette write line
(PB7) Bit 7 =Joy 3
9121 37153 Port A output register
keyboard row scan
9122 37154 Data direction register B
9123 37155 Data direction register A
9124 37156 Timer 1, low byte latch
9125 37157 Timer 1, high byte latch
9126 37158 Timer 1, low byte counter
9127 37159 Timer 1, high byte counter
timer 1 is used for the
60 time/second interrupt
9128 37160 Timer 2, low byte latch
9129 37161 Timer 2, high byte latch
912A 37162 Shift register
912B 37163 Auxiliary control register
912C 37164 Peripheral control register
CA1 Cassette read line (Bit 0)
CA2 Serial clock out (Bits 1-3)
CB1 Serial SRQ IN (Bit 4)
CB2 Serial data out (Bits 5-7)
912D 37165 Interrupt flag register
912E 37166 Interrupt enable register
912F 37167 Port A output register
9130-93FF Unused, due to the physical design of the VIC
9400-97FF 37888-38911 I/O BLOCK 1 (COLOR RAM)
9400-95FF 37888-38399 for 8k+ VIC: COLOR RAM
9600-97FF 38400-38911 for Unexpanded VIC: COLOR RAM
9800-9BFF 38912-39935 I/O BLOCK 2
9C00-9FFF 39936-40959 I/O BLOCK 3
A000-BFFF 40960-49152 BLK 5 – ROM expansion (cartridges) will be allocated here
C000-DFFF 49152-57343 BASIC – 8K ROM
E000-FFFF 57344-65535 KERNAL – 8K ROM

Adapted from Samm Laur’s post in comp.sys.cbm (1994)

Configurations Comparison Scheme

VIC-20 Handy Tips

  1. To find where BASIC Area starts: PEEK(43)+256*PEEK(44)
  2. To find where BASIC Area ends: PEEK(55)+256*PEEK(56)
    These can also be changed to protect memory from BASIC
  3. To find which “page” screen memory starts: PEEK(648)
    30 = $1E means $1E00 (Unexpanded or 3K expanded VIC)
    16 = $10 means $1000 (8k+ VIC)
  4. To quickly change memory size and reset while keeping keyboard buffer intact: POKE a new memory start into 641-642, then SYS64824
  5. To perform a machine reset: SYS64802

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