Secret Variables

If you've ever written software of any reasonable complexity (something more useful than "Hello World") then you're probably aware that software typically has many internal or "secret" variables. These are never (directly) exposed the user. This is, generally speaking, good software design: hiding the complexity of the software (i.e., hiding trivial variables) makes it easier to use and easier to upgrade. The CBM BASIC interpreter (really, almost any BASIC implementation) is software itself (which in this case lets you create BASIC software).

So on the one hand, "secret" variables should come as no suprise. On the other hand, CBM BASIC (indeed, every BASIC that I know) includes "secret" variables that are important to the programmer but not directly accessibly. These are usually "write-only" variables; which is to say BASIC allows you some way to change them, but BASIC provides no direct way to read them. (In rare cases, they are read-only or completely hidden.) This is bad because if you need to know it/their value(s) in a program, then you have only a few "bad" choices:

Use machine-specific PEEKs (bad because it is non-portable)
Maintain a normal variable ("shadow") that mimics the hidden variable (bad because this is very error-prone)
Use knowledge of BASIC's internal behavior to anticipate the value (bad because it may be non-portable AND is error-prone)

Most BASIC software I have seen relies on option #3. On the one hand, this is the worst method because it allows for the greatest chance of failure. I'm guessing the reason for its popularity is that it is the fastest method (because you are neither PEEKing nor maintaining a "shadow").

Below is a table listing some important "secret variables" of BASIC. Some versions of BASIC lack the features that need an associated "secret variable" and they are indicated with "n/a". Note that some of them are actually maintained by the KERNAL and not the BASIC interpreter. By looking at the table, you will see that virtually all secret variables are located at a machine-specific location, which makes their use non-portable (unless you include special detection code and act appropriately). The different values are listed under PEEK (POKE) Address; the column "CBM-II" refers to the "business" line of machines with various names in different countries (these machines are pretty easy to recognize because they power-up in lower/upper case PETSCII font, unlike all the others which default to upper/graphic ASCII-X font); the TED column refers to the C16 and Plus/4.

This is definately not a complete list! Many things maintained by the system are not important to a BASIC programmer (mainly temporary values and indirect pointers) and thus are not included. For example the BASIC Text Pointer is very important to the BASIC interpreter, but not very relevant to the BASIC programmer (mainly because it is constantly changing while a program runs). The same is true of the floating-point accumulator; so I would not normally include it in such a listing, but it is specifically needed by the USR function. In other words, I've tried to keep the list short, but with 110 entries (assuming I counted correctly) it seems I'm not good at picking relevant entries or CBM BASIC is littered with secret variables. Check out the list and decide for yourself!

Secret Variable	Type	PEEK (POKE) Address						Read method	Write method	Default Value	Note(s)
Secret Variable	Type	PET	CBM-II	VIC-20	C64	TED	C128	Read method	Write method	Default Value	Note(s)
CPU P register	Byte	?	none!	?	783	2037	5	RREG	SYS	0 ?	RREG/SYS access only with C128
CPU A register	Byte	?	none!	?	780	2034	6	RREG	SYS	0 ?	RREG/SYS access only with C128
CPU X register	Byte	?	none!	?	781	2035	7	RREG	SYS	0 ?	RREG/SYS access only with C128
CPU Y register	Byte	?	none!	?	782	2036	8	RREG	SYS	0 ?	RREG/SYS access only with C128
File#	Byte	?	26	?	19 ?	19 ?	21	none	CMD	0	0 = keyboard input and screen output
Start of Text "TxtTab"	Unsigned Word	40 ~ 41	45 ~ 46	?	43 ~ 44	43 ~ 44	45 ~ 46	none	GRAPHIC	varies	GRAPHIC only in v3.5 and v7.0
End of Text "TxtEnd"	Unsigned Word	n/a	47 ~ 48	n/a	n/a	n/a	4624 ~ 4625	FRE	CLR, DELETE, DLOAD, LOAD, NEW, RUN	varies	For machines with more than 64K RAM, this refers to Bank 0 (program text). For machines with 64K or less RAM, this is equivalant to Start of Variables. FRE() only gives a value indirectly.
Start of Variables "VarTab"	Unsigned Word	42 ~ 43	49 ~ 50	?	45 ~ 46	45 ~ 46	47 ~ 48	none	CLR, DELETE*, DLOAD, LOAD, NEW, RUN	varies	(D)LOAD does not update this when executed in a program. *DELETE only affects machines with 64K or less.
Limit of Scalars "VarEnd"	Unsigned Word	n/a	51 ~ 52	n/a	n/a	n/a	n/a	FRE(2)	CLR, DLOAD, LOAD, NEW, RUN	Start of variables	For machines with more than 192K. Other machines use Start of Arrays.
Start of Arrays (Limit of Scalars) "AryTab"	Unsigned Word	44 ~ 45	53 ~ 54	?	47 ~ 48	47 ~ 48	49 ~ 50	none	CLR, DELETE*, DLOAD, LOAD, NEW, RUN	Start of variables	(D)LOAD does not update this when executed in a program. *DELETE only affects machines with 64K or less.
Limit of Arrays "AryEnd"	Unsigned Word	n/a	55 ~ 56	n/a	n/a	n/a	n/a	FRE(3)	CLR, DLOAD, LOAD, NEW, RUN	Start of Arrays	For machines with more than 128K. Other machines use Free Space.
Free Space (Limit of Arrays) "StrEnd"	Unsigned Word	46 ~ 47	57 ~ 58	?	49 ~ 50	49 ~ 50	51 ~ 52	FRE	CLR, DELETE*, DLOAD, LOAD, NEW, RUN	Start of arrays +Size of arrays	(D)LOAD does not update this when executed in a program. FRE() only gives a value indirectly. *DELETE only affects machines with 64K or less.
Start of Strings "FreTop"	Unsigned Word	48 ~ 49	59 ~ 60	?	51 ~ 52	51 ~ 52	53 ~ 54	FRE	CLR, DELETE*, DLOAD, LOAD, NEW, RUN	End of Variables	(D)LOAD does not update this when executed in a program. Note this value grows "down" in memory. FRE() only gives a value indirectly. *DELETE only affects machines with 64K or less.
Limit of Variables "MemTop"	Unsigned Word	52 ~ 53	63 ~ 65	?	55 ~ 56	55 ~ 56	57 ~ 58	FRE	none	Start of Strings	For machines with over 64K RAM, this refers to the limit of variables (Bank 1 in the C128). For machines with 64K or less RAM, it is the limit of BASIC (program text plus variables). In either case, it is a memory limit. FRE() only gives a value indirectly
Limit of Text (Points to input buffer on CBM-II)	Unsigned Word	n/a	136 ~ 137	n/a	n/a	n/a	4626 ~ 4627	FRE	none	varies	This is for machines with more than 64K RAM; it refers to the limit of program text. In other words, should End of Text (attempt) to exceed this value, an OUT OF MEMORY error is generated. For machines with 64K or less RAM, this is equivalant to Start of Strings. In either case, they are a memory limit. FRE() only gives a value indirectly
Current Line#	Unsigned Word	?	66 ~ 67	?	57 ~ 58	57 ~ 58	59 ~ 60		COLLISION, ELSE, GOSUB, GOTO, LOOP, NEXT, RESUME, RETURN, RUN, THEN, TRAP	none	The value here is always valid (when a program is running) and is saved to another location (the reserved variable EL) when an error occurs. COLLISION, LOOP, NEXT, RETURN and TRAP update this value indirectly.
DATA line#	Unsigned Word	?	72 ~ 73	?	63 ~ 64	63 ~ 64	65 ~ 66	none	RESTORE, RUN	first line of program
DATA pointer	Unsigned Word	?	74 ~ 75	?	65 ~ 66	65 ~ 66	67 ~ 68	none	RESTORE, RUN	start of first line of program	RESTORE lineNumber will set this to the start of the specified lineNumber
Stop Line#	Unsigned Word	?	68 ~ 69	?	59 ~ 60	601 ~ 602	4608 ~ 4609	none	none	varies	Set to the current line# whenever the END or STOP statements are encountered or the user presses the STOP key. Intended for use with CONT.
Stop Address	Unsigned Word	?	70 ~ 71	?	61 ~ 62	603 ~ 604	4610 ~ 4611	none	none	varies	Save the value of TextPointer (actual RAM address) for use with CONT (see above).
Error Line#	Unsigned Word	n/a	664 ~ 665	n/a	n/a	?	4617 ~ 4618	EL, HELP	CLR, RESUME, any program error	65535	Saved Line# where last error occured. This is not really a secret because of "public" EL, however the related TRAP Address (see below) is a secret.
Error Address	Unsigned Word	n/a	666 ~ 667	n/a	n/a	1269 ~ 1270	4622 ~ 4623	none	any error	varies	Saves the value of TextPointer (actual RAM address) for use by HELP and RESUME. The address points to the token that generated the error.
TRAP Line#	Unsigned Word	n/a	662 ~ 663	n/a	n/a	1266 ~ 1267	4619 ~ 4620	none	TRAP	65280	Line# to 'GOSUB' when an error occurs. If the high-byte is 255 then error trapping is not enabled. This may be due to either no TRAP routine was set, or TRAP has been disabled (such as when an error first occurs, to prevent recursion).
Enable Trace	Byte	n/a	n/a	n/a	n/a	747	4463	none	TRON, TROFF	0	A non-zero value (TRON sets 255) enables "tracing" of a running program. A zero value (set by TROFF) disables program tracing.
Collision Line# Low Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4729 ~ 4731	none	COLLISION	none!	Holds the low-byte of the BASIC line# to be called (like GOSUB) when a VIC-II event occurs and COLLISION is enabled. The 3 events are: sprite-sprite collision, sprite-foreground collision, and light-pen triggered. Set by COLLISION. Used by the main BASIC loop (in program mode only).
Collision Line# High Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4732 ~ 4734	none	COLLISION	none!	Holds the high-byte of the BASIC line# to be called (like GOSUB) when a VIC-II event occurs; see above.
Collision Control	Byte	n/a	n/a	n/a	n/a	n/a	4735	none	COLLISION, enter RUN mode	0	The low 3 bits determine which type of VIC-II event(s) will cause a program to call a subroutine (like GOSUB). An event is enabled when a bit is set (1). All three bits are set by COLLISION. When the BASIC main loop actually calls the subroutine, bit 7 is set (to prevent recursion) and the next RETURN will clear bit 7. Cleared to zero when entering program mode (typically with RUN).
AUTO value	Unsigned Word	n/a	n/a	n/a	n/a	115 ~ 116	116 ~ 117	none	AUTO, CONT, GOSUB, GOTO, RUN	0	Every write method except AUTO sets the value to 0
USR Vector "UsrPok"	Unsigned Word	?	3 ~ 4	?	785 ~ 786	1281 ~ 1282	4633 ~ 4634	none	none	varies	This is a pointer to some machine language routine that runs whenever USR is called. The default routine will print an error message.
Seed	Packed Float	?	?	?	139 ~ 143	1283 ~ 1287	4635	none	RND	varies	This is a floating-point value used to generate the next random number when the argument to RND is zero. This value may be set by giving a negative argument to RND.
FAC (USR value)	Unpacked Float	?	113 ~ 118	?	97 ~ 102	97 ~ 102	99 ~ 104	many	many	none	This is the Floating Point Accumulator and is used by nearly all numeric operations of BASIC. In particular, it holds the argument to USR function, and the result of the function should be stored in FAC as well.
BASIC top-of-stack	Unsigned Word	n/a	n/a	n/a	n/a	124 ~ 125	125 ~ 126	none	DO, FOR, GOSUB, LOOP, NEXT, RETURN	C128: 2559, TED: 1968	Most versions of CBM BASIC use the CPU stack for DO / FOR / GOSUB.
CPU top-of-stack	Byte	n/a	668	n/a	n/a	1271?	130	none	everything, RESUME, RUN	varies	Almost everyting affects the CPU stack pointer; in particular, an error TRAP will (effectively) save the value, RESUME will (literaly) restore it, and RUN will reset it.
Current Bank	Byte	n/a	599	n/a	n/a	n/a	981	none	BANK	15	Holds the Bank# to be used by various commands: BOOT, BLOAD, BSAVE, FETCH, PEEK, POKE, STASH, SWAP, SYS, WAIT.
RUN Mode	Byte	?	67	?	?	129	127		CONT, END, GOSUB, GOTO, LOOP, NEXT, RETURN, RUN, STOP	0	A value of zero indicates direct mode; except CBM-II, a value less than 255 indicates RUN mode. See also Enter RUN Mode.
I/O Status	Byte	?	156	?	144	144	144	ST	varies	0	Updated by any non-keyboard or non-screen I/O; reading the reserved variable ST will reset this secret variable to zero
Enable	Byte	?	?	673?	673	1998? 2000?	2575	none	none	varies	Bit 0 (value 1) is set when transmitting RS-232 data
Number of OPEN files	Byte	?	864	?	152	151	152	none	APPEND, CLOSE, CLR, COLLECT, DCLEAR, DCLOSE, DOPEN, OPEN, RUN	0	(D)OPEN normally increases this value by 1; DOPEN a REL files increases it by 2; DCLEAR decreases this value based on the number files opened on the particular device; CLR and RUN reset the value to 0.
Input Device	Byte	?	161	?	153	152	153	none	GET#, INPUT#	0	Default is the keyboard
Output Device	Byte	?	162	?	154	153	154	none	CMD, PRINT#	3	Default is the (active) display screen
DosFA	Byte	n/a	544	n/a	n/a	631	284	none	Disk Commands	8?	Used by DS and DS$. On the C128, this is located in the cassette error pass log, so reading from cassette may foul-up DS or DS$... possibly locking up the system. On any system it is not updated by CLOSE, CMD, GET#, LOAD, OPEN, PRINT#, SAVE, or VERIFY.
DS Descriptor	String descriptor	n/a	22 ~ 25	n/a	n/a	?	122	(indirect) DS, DS$	Disk commands, CLOSE, CMD, GET#, INPUT#, LOAD, OPEN, PRINT#, SAVE, VERIFY	null descriptor	This describes a string which is the most recently read "disk" error/status message (actually any device 8 to 11, channel 15). The descriptor is cleared (null string) for any of the listed write methods. Some of the disk commands will later (after null) read a new string from the device. It is referenced by DS and DS$; if the descriptior is null, the error/status channel is read into a new string.
Message Mode	Byte	?	?	?	157	129	157	none	error, CONT, END, GOSUB, GOTO, RUN, STOP	0 (RUN mode) 128 (direct mode)	Enables messages such as SEARCHING FOR and LOADING. When you enter the built-in MONITOR, this value is set to 192, which results in cryptic IO ERROR numbers in some cases.
Start Address	Unsigned Word	n/a	?	n/a	n/a	178 ~ 179	172 ~ 173	none	BOOT, BLOAD, BSAVE, DLOAD, DSAVE, DVERIFY, LOAD, SAVE, VERIFY	varies	BOOT, BLOAD, BSAVE and the ML-versions of LOAD and VERIFY set this value based on the value in the file header by default (BOOT, BLOAD, BSAVE have parameters to specify a specific value). The BASIC-versions of LOAD, SAVE, and VERIFY set it to the Start of Text.
End Address (plus 1)	Unsigned Word	?	?	?	174 ~ 175	157 ~ 158	174 ~ 175	none	BOOT, BLOAD, BSAVE, DLOAD, DSAVE, DVERIFY, LOAD, SAVE, VERIFY	varies	The listed write methods set the value to the last address loaded/saved/verified plus one.
Last Device	Byte	?	159	?	186	174	186	none	varies	0	Set by the most recent I/O command (except PRINT, GET, GETKEY, and INPUT)
RS-232 Input Buffer Pointer	Unsigned Word	?	?	?	247 ~ 248	?	200 ~ 201	none	CLOSE, OPEN	varies	*On the C128, the default is 3072 ($C00); Dynamically set on C64 by OPEN and CLOSE.
RS-232 Output Buffer Pointer	Unsigned Word	?	?	?	249 ~ 250	?	202 ~ 203	none	CLOSE, OPEN	varies	*On the C128, the default is 3328 ($D00); Dynamically set on C64 by OPEN and CLOSE.
Cursor Row	Byte	?	202	?	214	205	235	none	varies	none	Anything that writes to the screen can update this value. CHAR can set a specific value.
Cursor Column	Byte	?	203	?	211	202	236	POS	varies	none	Anything that writes to the screen can update this value; only TAB() and CHAR can set a specific value.
Cursor Off	Byte	?	?	?	204	1340	2599	none	END, INPUT, STOP	varies	A zero value enables the text screen cursor (not C128 80-column mode); a non-zero value hides it. In direct mode, or during program INPUT, the value is 0 so the user can see where (s)he is typing. A non-zero value is present during normal program execution so the cursor is hidden.
Bottom of Window	Byte	n/a	221	n/a	n/a	2021	228	none	CMD, PRINT, PRINT#, WINDOW	18	A zero-based physical line number. Can be set with WINDOW command or by printing "ESC B" [CHR$(27);"B"].
Top of Window	Byte	n/a	220	n/a	n/a	2022	229	none	CMD, PRINT, PRINT#, WINDOW	0	A zero-based physical line number. Can be set with WINDOW command or by printing "ESC T" [CHR$(27);"T"].
Left of Window	Byte	n/a	222	n/a	n/a	2023	230	none	CMD, PRINT, PRINT#, WINDOW	0	A zero-based physical column number. Can be set with WINDOW command or by printing "ESC T" [CHR$(27);"T"].
Right of Window	Byte	n/a	223	n/a	n/a	2024	231	none	CMD, PRINT, PRINT#, WINDOW	39 (most), 79 (C128 80-column)	A zero-based physical column number. Can be set with WINDOW command or by printing "ESC T" [CHR$(27);"T"].
Vertical Wrap	Byte	n/a	923	n/a	658 ?	2025	248	none	CMD, PRINT, PRINT#	0	A value of 128 or more enables vertical wrap: printing past the last line of the screen will resume on the top screen line. A value less than 128 enables the classic behavior: the text scrolls up one (or more) lines when printing goes past the last screen line. Printing "ESC M" [CHR$(27);"M"] will enable vertical wrap, while "ESC L" [CHR$(27)"L"] sets normal line scrolling.
Text Attribute	Byte	?	?	?	646 ?	1339	241	RCLR, RLUM	CMD, COLOR, PRINT, PRINT#	varies	RCLR only returns the color; on the TED series, RLUM returns the luminance. Other attribtues may present: Flashing characters for the TED series, and C128 80-column mode may include Flashing, Underline, Reverse, and Alternate Font bits. COLOR can't change the non-color (or non-luminance) attributes. CMD/PRINT# can only write this value if the fileNumber refers to the screen.
Rvs On	Byte	?	919	?	199	194	243	none	CMD, PRINT, PRINT#	0	A non-zero value enables reverse-font. Set to non-zero by printing "Reverse on" CHR$(18). Clear to zero by printing "Reverse off" CHR$(146) or [Shift]Return CHR$(13) [or CHR$(147)].
Quote Mode	Byte	?	210	?	212	203	244	none	CMD, PRINT, PRINT#	0	A non-zero value enables "quote mode" where control characters appear as reverse-font characters. This value is toggled on/off by printing (or typing) a double-quote ("), CHR$(34). It is zeroed by [Shift]Return, CHR$(13) [or CHR$(147)].
Insert Mode	Byte	?	211	?	216	207	245	none	CMD, PRINT, PRINT#	0	A non-zero value enables "insert mode" where control characters appear as reverse-font characters (similar to quote mode). This value is increased by each printing (or typing) an INSert code, CHR$(148) and is decreased (until 0) by printing/typing any other character. It is zeroed by [Shift]Return, CHR$(13) [or CHR$(147)].
Auto-Insert	Byte	n/a	922	n/a	n/a	2026	246	none	CMD, PRINT, PRINT#, RUN	0	A non-zero value enables "auto-insert mode". Unlike (manual) "Insert Mode", control characters operate normally. Printing "ESC A" [CHR$(27);"A"] sets this to non-zero. Printing "ESC C" [CHR$(27);"C"] or or entering RUN mode clears it to zero.
Scroll Lock	Byte	n/a	?	n/a	n/a	240	2593	none	none	0	A non-zero value will prevent output to the screen, effectively halting a program using PRINT. Unlike similar flags dealing with screen output, there are no control codes to change this value! Without resorting to POKE, only the user can toggle the value on/off with Control+S key combination or (C128) the NO SCROLL key.
Repeat Mode	Byte	?	?	?	650	1344	2594	none	none	varies	Controls which keyboard keys will repeat when held down: 0 = few (space, cursor, insert, delete), 64 = none, 128 = all.
Font Lock	Byte	?	?	?	657 ?	1351	247	none	CMD, PRINT, PRINT#	0	A non-zero value locks the current font, so it can not be changed by pressing C= and Shift keys (font can still be changed with other methods). The CHR$ code needed to enable / disable the lock varies by machine. See the ASCII-X or PETSCII tables.
Disable Bell	Byte	n/a	?	n/a	n/a	n/a	249	none	CMD, PRINT, PRINT#	0	(C128 Only) A value of 128 or more will disable the "ASCII Bell", otherwise it is enabled (a little beep generated when Control-G = CHR$(7) is printed or typed). Printing "ESC H" [CHR$(27);"H"] will set this to 128 and disable the bell; while "ESC G" [CHR$(27);"H"] will clear this and enable the bell.
Last Character	Char	n/a	219	n/a	n/a	2027	240	none	varies	none	Anything that writes to the screen updates this value; used to recognize ESCape key sequences.
Keyboard Buffer	Byte[10]	?	939 ~ 948	?	631 ~ 640	1319 ~ 1328	842 ~ 851	GET, GETKEY, INPUT	none		Keys pressed by the user are stored here until they can be processed. Some programs will PRINT instructions on the screen and POKE a Return-key-code (or multiple Returns) here and then END so that BASIC will execute the PRINTed instructions as if the user had typed them. Needed for some BASIC statements that "fail" in program mode.
Number of buffered keys	Byte	?	209	?	198	239	208	none	GET, GET#, GETKEY, INPUT, INPUT#	0	GET# and INPUT# only write a new value if the fileNumber refers to the keyboard
Shift flag	Byte	?	224	?	653	1348	211	none	none	0	Indicates which (if any) of ALT, C=, CAPS LOCK, CTRL, or SHIFT[lock] key(s) are currently pressed
Key Code	Byte	?	225	?	197 ?	2038	212	GET, GET#, GETKEY, INPUT, INPUT#	none	64 (most) 88 (C128)	A machine-specific code indicating which key is currently is pressed; if none the default value is present. Some keys (like RESTORE and SHIFT) do not update this. The listed read methods will translate the Key Code into a PETSCII value, but they will return the value of any buffered keys first, and GET# / INPUT# only apply if the fileNumber refers to the keyboard.
Function Key Lengths	Byte[varies]	n/a	899 ~ 918	n/a	n/a	1375 ~ 1382	4096 ~ 4105	none	KEY	varies	See note below
Function Key Definitions	Byte[varies]	n/a	64512 ~ 65022	n/a	n/a	1383 ~ 1511	4106 ~ 4351	none	KEY	varies	There is no real way to get the values; KEY can be used to display them, but the result can't be used (like assigned to a variable).
PRINT USING Virtual Characters "PUchrs"	Char[4]	n/a	627 ~ 630	n/a	n/a	1255 ~ 1258	4612 ~ 4615	none	PUDEF	Space, Comma, Period, Dollar	These are the actual characters that will be used by PRINT USING when it needs to print a corresponding virtual character. For example, you can use PUDEF to change the monetary symbol from the default dollar ($) to British Pound (£) or some other character. However BASIC provides no way to determine the current assignment(s).
Screen Start	Byte	?	?	?	648	1342	2619	none	none	4 (many) 12 (TED)	The high-byte of the start of the text screen; used by the screen editor. To relocate the text screen in RAM, not only this value, but chip register(s) must be changed too. On the C128, this only applies to the 40-column screen.
Screen Start 80	Byte	n/a	n/a	n/a	n/a	n/a	2606	none	none	0	(C128 Only) The high-byte of the start of the 80-column text screen; used by the screen editor. To relocate the text screen in RAM, not only this value, but chip registers must be changed too
Color Start 80	Byte	n/a	n/a	n/a	n/a	n/a	2607	none	none	8	(C128 Only) The high-byte of the start of the 80-column attributes; used by the screen editor. To relocate the attributes in RAM, not only this value, but chip registers must be changed too
PAL / NTSC	Byte	?	?	?	678	?	2563	none	none	varies	Zero indicates an NTSC system; non-zero indicates (most) PAL systems. Unknown value for the rare "PAL-M" system. This may be important for using reserved variable TI or generating SOUND.
Bitmap allocated	Byte	n/a	n/a	n/a	n/a	117	118	none	GRAPHIC	0
Split Raster	Byte	n/a	n/a	n/a	n/a	n/a	2612	none	GRAPHIC	208	(C128 Only) This value determines the start of the text-mode part of a split-screen. Value = 48 + 8 * Text_Row (0-based row#). Although the Plus/4 has split-screen, the text part appears at a fixed location.
SCALE X	Unsigned Word	n/a	n/a	n/a	n/a	n/a	137 ~ 138	none	SCALE	20480 (high-res) 10240 (multi-color)
SCALE Y	Unsigned Word	n/a	n/a	n/a	n/a	n/a	139 ~ 140	none	SCALE	12800
Enable SCALE	Byte	n/a	n/a	n/a	n/a	742	4458	none	SCALE	0	A non-zero value enables bitmap coordinate scaling. A zero value treats bitmap coordinates a literal pixel locations.
Bitmap line width	Byte	n/a	n/a	n/a	n/a	743	4459	none	WIDTH	0	A non-zero value causes a "dot" to be plotted on the bitmap as two pixels horizontally (this applies to drawing lines, circles, and boxes, not just random dots). A zero value draws only one pixel for each coordinate used in a bitmap drawing command.
Sprite pointers	Byte[8]	n/a	n/a	n/a	2040 ~ 2047	n/a	Text: 2040 ~ 2047 Bitmap: 8184 ~ 8191	none	none	56 ~ 63	These pointers determine the address where SPRITE data is stored (one pointer for each sprite). On the C128, there are actually two sets of pointers (one for text mode, one for bitmap mode) although they both point to the same locations. On the C64 the values are not defined but need to be set in order to use sprites.
Sprite data	Byte[8,64]	n/a	n/a	n/a	varies	n/a	3584 ~ 4095	SPRSAV	SPRSAV	none	On the C128, this area is the default area for sprite data storage; on the C64 the user must select an appropriate location(s). The SPRSAV command can be used to read/write the data to BASIC strings, but not read/write them to a disk file (use BLOAD / BSAVE instead).
Sprite vector table	Byte[8,11]	n/a	n/a	n/a	n/a	n/a	4478 ~ 4565	RSPPOS	MOVSPR	varies	This table controls the position and movement of each SPRITE (88 bytes, 11 for each of 8 sprites). The values can be set with MOVSPR, and for moving sprites the values are updated by the BASIC IRQ routine. The RSPPOS function can retrieve most of the values; but not the direction of movement. The horizontal direction is determined by an unsigned word beginning at offset 3; the vertical direction is determined by an unsigned word beginning at offset 5; and the sign of those two is determined by a code at offset 2 (0=NE, 1=SE, 2=SW, 3=NW).
Alternate Font Start	Byte	n/a	n/a	n/a	n/a	?	4587	none	none	216	High-byte of start of font to be used by CHAR (when used on a bitmap). By default it points to the upper/lower (PETSCII) font. The value here may be selected by using CHR$(14) in the CHAR string.
Primary Font Start	Byte	n/a	n/a	n/a	n/a	?	4588	none	none	208	High-byte of start of font to be used by CHAR (when used on a bitmap). By default it points to the upper/graphic (ASCII-X) font. The value here is the default used by CHAR; it may be manually selected by using CHR$(142) in the CHAR string. By POKEing this location, you make CHAR use your own custom font.
Tempo Rate	Byte	n/a	n/a	n/a	n/a	n/a	4642	none	TEMPO	16	This determines the speed of notes during PLAY. The default works out to about 3 quarter notes per second.
Play Remainders	Word[3]	n/a	n/a	n/a	n/a	n/a	4643 ~ 4648	none	PLAY	-256	Remaining time for each of the 3 SID voices. Set by PLAY and updated by the Tempo Rate during the BASIC IRQ. The note ends when the value "underflows" (becomes negative).
Octave	Byte	n/a	n/a	n/a	n/a	n/a	4651	none	PLAY	4	Determines note frequency. Set by PLAY octave directive. Should be 0 to 6.
Current Waveform	Byte[3]	n/a	n/a	n/a	n/a	n/a	4656	none	ENVELOPE, PLAY	65	This is the gated waveform for one of the 3 corresponding SID registers. The value is here is copied by PLAY to the SID chip (derived from ENVELOPE). The value minus 1 is copied by BASIC IRQ to the SID chip when the note ends (technically the value AND 254).
Attack Table	Byte[10]	n/a	n/a	n/a	n/a	n/a	4671 ~ 4680	none	ENVELOPE	varies	Holds the SID Attack/Decay chip register settings for each of 10 instruments (selected by PLAY instrument "tone" directive).
Sustain Table	Byte[10]	n/a	n/a	n/a	n/a	n/a	4681 ~ 4690	none	ENVELOPE	varies	Holds the SID Sustain/Release chip register settings for each of 10 instruments (selected by PLAY instrument "tone" directive).
Waveform Table	Byte[10]	n/a	n/a	n/a	n/a	n/a	4691 ~ 4700	none	ENVELOPE	varies	Holds the SID waveform chip register settings for each of 10 instruments (selected by PLAY instrument "tone" directive).
Pulse-Wdith Low Table	Byte[10]	n/a	n/a	n/a	n/a	n/a	4701 ~ 4710	none	ENVELOPE	varies	Holds the SID Pulse Width (low byte) chip register settings for each of 10 instruments (selected by PLAY instrument "tone" directive).
Pulse-Width High Table	Byte[10]	n/a	n/a	n/a	n/a	n/a	4711 ~ 4720	none	ENVELOPE	varies	Holds the SID Pulse Width (high byte) chip register settings for each of 10 instruments (selected by PLAY instrument "tone" directive).
Filter Frequency	Unsigned Word	n/a	n/a	n/a	n/a	n/a	4721 ~ 4722	none	FILTER	none!	Holds the SID registers setting for filter frequency (maximum is 2047). It has the last-used value from FILTER, it is never initialized.
Filter Resonance	Byte	n/a	n/a	n/a	n/a	n/a	4723	none	FILTER	none!	Holds the SID register setting for filter resonance (upper 4 bits) and voice application (lower 3 bits). It has the last-used value from FILTER, it is never initialized.
Filter / Volume	Byte	n/a	n/a	n/a	n/a	n/a	4724	none	FILTER, VOL	15	Holds the SID register settings for filter mode (bits 4~6) and master volume (lower 4 bits). The high bits are set by FILTER. The lower four bits are set by VOL and the PLAY "u" directive.
Sound Remainder Low Bytes	Byte[2 or 3]	n/a	n/a	n/a	n/a	1276 ~ 1277	4738 ~ 4740	none	SOUND	none!	Remaining time for each of the 3 audio voices (the TED only has 2 voices). Set by SOUND and decremented each BASIC IRQ. The value from this table, and a corresponding one from the next determine a 16-bit signed word.
Sound Remainder High Bytes	Signed Byte[2 or 3]	n/a	n/a	n/a	n/a	1278 ~ 1279	4741 ~ 4743	none	SOUND	-1	Remaining time for each of the audio voices. Set by SOUND and decremented by the BASIC IRQ until the value "underflows", and then the SOUND is silenced.
Sound Maximum Frequency Low Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4744 ~ 4746	none	SOUND	none!	Maximum frequency for each of the 3 audio voices. Set by SOUND and possibly checked each BASIC IRQ. These are the low bytes of an unsigned 16-bit word.
Sound Maximum Frequency High Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4747 ~ 4749	none	SOUND	none!	Maximum frequency for each of the 3 audio voices (see above); these are the high bytes.
Sound Minimum Frequency Low Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4750 ~ 4752	none	SOUND	none!	Minimum frequency for each of the 3 audio voices. Set by SOUND and possibly checked each BASIC IRQ. These are the low bytes of an unsigned 16-bit word.
Sound Minimum Frequency High Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4753 ~ 4755	none	SOUND	none!	Minimum frequency for each of the 3 audio voices (see above); these are the high bytes.
Sweep Modes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4756 ~ 4758	none	SOUND	none!	Determines how Sound Frequency is modulated; 0 = increase then reset, 1 = decrease then reset, 2 = increase then decrease, 3 =decrease then increase. Set by SOUND and tested each BASIC IRQ.
Sound Step Low Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4759 ~ 4761	none	SOUND	none!	Determines moulation amount (step size) for each of the 3 audio voices. Set by SOUND and possibly used each BASIC IRQ to update Sound Frequency.
Sound Step High Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4762 ~ 4764	none	SOUND	none!	Determines moulation amount (step size) for each of the 3 audio voices (see above); these are the high bytes.
Sound Frequency Low Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4765 ~ 4767	none	SOUND	varies	Current audio chip freqency register value for each of the 3 audio voices. Set by SOUND and modulated each BASIC IRQ by the "step size" according to the "sweep mode". The value from this table, and a corresponding one from the next determine a 16-bit unsigned word.
Sound Frequency High Bytes	Byte[3]	n/a	n/a	n/a	n/a	n/a	4768 ~ 4770	none	SOUND	varies	Current audio chip freqency register value for each of the 3 audio voices (see above); these are the high bytes.
Sound Pulse Width	Unsigned Word	n/a	n/a	n/a	n/a	n/a	4782 ~ 4783	none	SOUND	none	Value of pulse-width used in the most recent SOUND statement.
Sound Waveform	Byte	n/a	n/a	n/a	n/a	n/a	4784	none	SOUND	varies	Waveform setting for an audio chip register used in the most recent SOUND statement.