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Commodore VIC-20 Memory Map

Newsgroups: comp.sys.cbm
From: slaur@utu.fi (Sam Laur)
Subject: VIC-20 memory map (long)
Date: Wed, 2 Mar 1994 12:54:14 GMT

Okay, here's what all you VIC-20 freaks out there have been waiting for... Sorry for the delay, but I guess I'm just too lazy to correct the errors that OCR scanners make :-) Also I found a couple of typos in the VIC-20 Programmer's Reference Guide itself. I tried to correct everything, but who knows, there may still be quite many typos lurking in this. Here it comes:


MEMORY MAPS

The following memory maps provide a guide which shows which special locations are set aside for use by the VlC's operating system . . . and what those locations are used for.

* Useful memory location
  HEX      DECIMAL  DESCRIPTION
  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
  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
*002D-002E  45-46    Pointer: Start of Variables
*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)
0035-0036   53-54    Utility string pointer
*0037-0038  55-56    Pointer: Limit of memory
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    Input vector
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       Accum#1: Exponent
*0062-0065  98-101   Accum#1: Mantissa
 *0066     102      Accum#1: Sign
  0067     103      Series evaluation constant pointer
  0068     104      Accum#1 hi-order (overflow)
*0069-006E  105-110  Accum#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)
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      How many open files
 *0099     153      Input device (normally 0)
 *009A     154      Output (CMD) device, normally 3
  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  Jiffy Clock (HML)
  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
  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      # characters in file name
 *00B8     184      Current logical file
 *00B9     185      Current secondary address
 *00BA     186      Current device
*00BB-00BC  187-188  Pointer: to file name
  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 pressed
 *00C6     198      # chars in keyboard buffer
 *00C7     199      Screen reverse flag
  00C8     200      Pointer: End-of-line for input
00C9-00CA   201-202  Input cursor log (row, column)
 *00CB     203      Which key: 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
 *00D3     211      Position of cursor on above line
  00D4     212      0=direct cursor, else programmed
 *00D5     213      Current screen line length
 *00D6     214      Row where cursor lives
  00D7     215      Last inkey/checksum/buffer
 *00D8     216      # of INSERTs outstanding
*00D9-00F0  217-240  Screen line link table
  00F1     241      Dummy screen link
  00F2     242      Screen row marker
*00F3-00F4  243-244  Screen color pointer
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  Operating system free zero page space
00FF        255      Basic storage
0100-010A   256-266  Floating to ASCII work area
0100-013E   256-318  Tape error log
0100-01FF   256-511  Processor stack area
*0200-0258  512-600  Basic input buffer
*0259-0262  601-610  Logical file table
*0263-026C  611-620  Device # table
*026D-0276  621-630  Secondary Address table
*0277-0280  631-640  Keyboard buffer
*0281-0282  641-642  Start of memory for op system
*0283-0284  643-644  Top of memory for op system
0285        645      Serial bus timeout flag
*0286       646      Current color code
0287        647      Color under cursor
*0288       648      Screen memory page
*0289       649      Max size of keyboard buffer
*028A       650      Key repeat (128=repeat all keys)
*028B       651      Repeat speed counter
028C        652      Repeat delay counter
*028D       653      Keyboard Shift/Control flag
028E        654      Last keyboard shift pattern
028F-0290   655-656  Pointer: decode logic
*0291       657      Shift mode switch (0 = enabled,  128-
                    locked)
0292        658      Autoscrolldownflag (0=on)
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  Error message link
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                 (FED2)
0330-0331    816-817       Link to load RAM            (F549)
0332-0333    818-819       Link to save RAM            (F685)
0334-033B    820-827       ??
*003C-03FB   828-1019      Cassette buffer
0400-0FFF    1024-4095     3K expansion RAM area
1000-1DFF    4096-7679     User Basic area
1E00-1FFF    7680-8191     Screen memory
2000-3FFF    8192-16383    8K expansion RAM/ROM block 1
4000-5FFF    16384-24575   8K expansion RAM/ROM block 2
6000-7FFF    24576-32767   8K expansion RAM/ROM block 3
NOTE: When additional memory is added to block 1 (and 2 and 3), the
KERNAL relocates the following things for BASIC:
1000-11FF    4096-4607     Screen memory
1200-?       4608-?        User Basic area
9400-95FF    37888-38399   Color RAM
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
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 8x8 or 16x8 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 startinq address
                                       HEX   DEC
                          0000   ROM  8000  32768
                          0001        8400  33792
                          0010        8800  34816
                          0011        8C00  35840
                          1000   RAM  0000  0000
                          1001        xxxx
                          1010        xxxx  unavail.
                          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 sets volume of all sound
                 bits 4-7 are 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
          ID     ID
          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
9400-95FF   37888-38399        location of COLOR RAM with
                               additional RAM at blk 1
9600-97FF   38400-38911        Normal location of COLOR RAM
9800-9BFF   38912-39935        I/O block 2
9C00-9FFF   39936-40959        I/O block 3
A000-BFFF   40960-49152        8K decoded block for expansion ROM
C000-DFFF   49152-57343        8K Basic ROM
E000-FFFF   57344-65535        8K KERNAL ROM
USEFUL MEMORY LOCATIONS
This is a more in-depth guide to some of the memory locations
you can use.
HEX         DECIMAL  DESCRIPTION
0014-0015   20-21    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.
002B-002C   43-44    The start of the BASIC program in
                    memory. 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    The start of the numeric variables,
                    which is usually immediately after the
                    end of the BASIC program.
002F-0030   47-48    The start of arrays in memory, usually
                    immediately following the numeric vari-
                    ables.
0031-0032   49-50    The end of the arrays in memory.
0033-0034   51-52    Bottom of string storage, moving from
                    the top of available memory down to the
                    top of arrays.
0037-0038   55-56    The top of free RAM. By lowering this
                    value, some RAM can be "protected"
                    against BASIC putting values here.
0043-0044   67-68    Jump vector for INPUT statement.
0061-0066   97-102   Floating point accumulator #1 for cal-
                    culations.
0069-006E   105-110  Floating point accumulator #2.
0073-008A   115-138  The CHRGET subroutine resides here.
                    This routine gets the next BASIC
                    character from machine language.
0090        144      Status word ST.
0098        152      Number of open files.
0099        153      Device number for input, normally 0
                    (keyboard).
009A        154      Output (CMD) device, normally 3
                    (screen).
00A0-00A2  160-162  3 byte jiffy clock. The Tl and Tl$
                   variables are translations of these loca-
                   tions.
00B2-00B3  178-179  Points to the start of the tape buffer. Can
                   be used as an indirect zero-page jump
                   to a routine in the buffer.
00B7       183      Number of characters in filename.
00B9       185      Which secondary address is currently
                   being used.
00BA       186      Current device number being accessed.
00BB-00BC  187-188  Points to location of filename in memo-
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  £ (pound)    22 ;            38 none         54 ^ (up arrow)
7  DEL          23 crsr lt/rt   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 crsr up/dn   47 f3           63 f7
00C6       198      Number of characters currently in key-
                   board buffer.
00C7       199      Flag for reverse on/off. A 1 here is on, a
                   0 is off.
00CB       203      Same as 197.
00D1-00D2  209-210  Address of start of line where cursor is.
00D3       211      Position of cursor on line.
00D5       213      Current screen line length--either 21,
                   43, 65, or 87.
00D6       214      Screen row where cursor is. To change
                   the cursor position, locations 209, 210,
                   211, and 214 must be changed.
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.
00F3-00F4  243-244  Pointer to the current space in color
                   memory.
00FB-00FE  251-254  Available locations in zero page.
0200-0258  512-600  BASIC input buffer--where the charac-
                   ters 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  Start of memory pointer.
0283-0284  643-644  Top of memory pointer.
0286       646      Current color code. This holds the color
                   number that goes into color memory
                   during PRINT operations.
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      Maximum 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      This determines how long the VIC waits
                   before repeating key.
028D       653      Keyboard SHIFT, CTRL, Commodore
                   flag. The SHIFT sets the 1 bit, Commo-
                   dore sets the 2 bit, and the CTRL sets
                   the 4 bit.
0291            657    Setting this location to 128 will disable
                      switching case, and a 0 here enables
                      the ability to switch.
0300-0301     768-769  This is the jump vector for errors. By
                      changing this vector, a routine can
                      intercept any error condition.
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 pro-
                      grams.


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