Changes

The are two versions of Locomotive BASIC used in the Amstrad CPC and CPC+ computers. v1.0 is used by the CPC464, and v1.1 is used by the CPC664, CPC6128, CPC464+ and CPC6128+[[Category:Cpctech.org]]

A RSX (Resident System Extension) command is accessed through = Technical information about Locomotive BASIC with a "|" character prefix. e.g. The "DIR" RSX, is accessed by "|DIR". =

A binary function is accessed through The are two versions of Locomotive BASIC using used in the Amstrad CPC and CPC+ computers. v1.0 is used by the CPC464, and v1.1 is used by the "CALL" commandCPC664, CPC6128, CPC464+ and CPC6128+.

BASIC v1.0 There is a$="A"region of memory between AC00-AC1B which is filled with RET instructions. The BASIC ROM calls the following locations:|DRIVEAC01,@a$AC04,AC07,AC0A,AC0D,AC10,AC13,AC16,AC19

BASIC v1It almost looks like these were used for debugging the ROM but were left in.1:

== Additional keywords and variables in BASIC v1.1 Passing parameters to a RSX (Resident System Extension) command or binary function ==

* A RSX (Resident System Extension) command is accessed through BASIC v1with a "|" character prefix.1 has e.g. The "DIR" RSX, is accessed by "|DIR". A binary function is accessed through BASIC using the following additional keywords:"CALL" command.

Both RSX and CALL commands work (are!) similar from the BASIC command line and invoke machine code - the only difference is:* COPYCHR$with the help of a RSX command you don't need to know the exact access address. You can pass up to 32 parameters with a CALL or RSX command. Possible parameters could be integer, floating points and strings. Just store the parameter into a variable:* CURSORPassing for integer:* DEC$:* FILL<pre>:* FRAMECALL &4000,32,34,&5E,&x10101010:* GRAPHICS|RSX-command,32,34,&5E,&x10101010:* MASK </pre>Passing for floating points<pre>@a!=43.375CALL or |RSX-command,@a!</pre>

* <br> Passing a string parameter: <br> BASIC v1.0 <pre>a$="A":|DRIVE,@a$</pre> BASIC v1.1 has the following additional predefined variables:<pre>|DRIVE,"A"</pre>or<pre>b$="test"CALL or |RSX-command,@b$</pre>

The parameters will be stored inside the range of Stack:* DERR <pre> A=n -------> MSB PARAM1 LSB PARAM1 MSB PARAM2 LSB PARAM2 ... ... ... MSB PARAMn IX+0 ------> LSB PARAMn</pre>with the help of that assembly code is it possible to find the parameters:<pre>ROUT1: CP x ;test register A if "x" parameters were given, e.g. x = 2 RET NZ ; if not exactly x parameters then do not execute RSX. LD L,(IX+0) LD H,(IX+1) ; HL = value of last parameter LD E,(IX+2) LD D,(IX+3) ; DE = value of next to last parameter ...</pre>A register holds the number of parameters. IX points to each parameter. IX+0/IX+1 is the last parameter. This happens because each parameter in order is pushed onto the stack and IX then points to the last parameter pushed. Each parameter is a 16-bit value.

If a parameter is a string, then IX holds the address of the string descriptor which point to a 3 byte block. The first byte of the string descriptor is the length. The next 2 bytes are an address that point to the string in memory. With the help of the variable container "@" it is also possible to get a result from an invoked mc-code back to basic.In this case the parameter contains the address of the integer, floating point or string. You can modify these to give the result.BASIC allocates the strings, so when you modify a string, do not write more characters than the original string. Hint: the first two bytes at the end of stack shows the internal return address from BASIC interpreter where it was executed. == Additional keywords and variables in BASIC v1.1 == #BASIC v1.1 has the following additional keywords: #*COPYCHR$ #*CURSOR #*DEC$ #*FILL #*FRAME #*GRAPHICS #*MASK #BASIC v1.1 has the following additional predefined variables: #*DERR <br> <br>  == Entering BASIC programs ==

There are some rules for assigning names for variables:: - it is not allowed to define a variable starting with a figure (the system expect then programming mode and interpret it as a program line number): - no space (or 'under' as it is common today) between variable names are allowed: - BASIC keywords can't be used: - different sign e.g.: a slash (doesn't depend which direction), a 'minus' sign, a 'and' sign can't be used in a variable names: - after a dollar sign (for defining string variables) it is not allowed to use further signs: - different defining functions together are not allowed (e.g. use '%' together with '$') : - you can use 40 characters for a variable name: - use ! for defining a REAL value variable (like the function DEFREAL but only for the assigned variable) > It's also possible to omit the exlamation mark because it's the default system declaration after powering on then CPC.: - use % for defining a INTEGER value variable (like the function DEFINT but only for the assigned variable): - use $ for defining a STRING variable (like the function DEFSTR but only for the assigned variable): - Minimum line number is 1. Maximum line number is 65535. This is converted into the tokenised BASIC program which is more compact than the line entered. The BASIC keywords are converted into "tokens", or 1-2 byte sequences which uniquely identify each keyword.  <br> == Speeding up BASIC programs == To speed up BASIC programs following rules could help:: - Avoid loops or a branching program structure if possible. Internally the operating system searches for the right line number always from the beginning of the program 'listing'. Often recurring subroutines (if necessary) should be at the beginning of RAM to speed up things.: - Avoid the index variable after the <code>NEXT</code> command/statement (although it isn't a good program style): - Avoid spaces inside program code or remarks: - Put lots of statements as possible in ONE line: - Use variables instead of constants. A constand has always to be converted in a binary/digital format. A variable is already converted.: - Dimensioning at the beginning of the program: - Dimensioning your variables & pre-calculate: - Avoid AND/OR/XOR in IF-THEN statements. It's faster to write IF ... THEN (statement 1).. IF ... THEN (statement 2).

== Structure of a BASIC program ==

A BASIC program is stored in a tokenised format. Here the keywords are represented by 1 or 2 byte unique sequences.Each line of the BASIC program has the form:

{|{{Prettytable|width: 700px; font-size: 2em;}}|''Offset''||''Size''||''Description''border="1"

|2|0 |2||16-bit decimal integer Length of line number data in bytes (note 21)

|42 |2 |n||BASIC 16-bit decimal integer line encoded into tokens number (note 32)

| 4 |n+1||1||"0" the end of BASIC line marker encoded into tokens (note 43)

'''<br> Notes:'''

* #This 16-bit value has two functions::* if "0" it signals the end of the BASIC program. In this case, there is no furthur BASIC program lines or data.:* , otherwise, this value defines the length of the tokenised BASIC program line in bytes, and includes the 2 bytes defining this value, the 2 bytes defining the program line number, and the end of line marker. This number is stored in little endian notation with low byte followed by high byte. * #This 16-bit value defines the line number and exists if the length of line data in bytes is not "0". A line number is a integer number in the range 1-65535. This number is stored in little endian notation with low byte followed by high byte.* #This data defines the tokenised BASIC program line and exists if the length of line data in bytes is not "0". The length is dependant on the BASIC program line contents.* #This value defines the end of the tokenised BASIC line data and exists if the length of line data in bytes is not "0". The BASIC interpreter looks for this token during the processing of this line, and if found, will stop execution and continue to the next line.

== BASIC tokens ==

This table list the BASIC tokens with no prefix byte.

<br>  {|{{Prettytable|width: 700px; font-size: 2em;}}|''Offset''||''Count''||''Description''border="1"

|1&amp;00 ||1||8-bit byte offset to tokens following RSX name.end of tokenised line marker

|2&amp;01 ||x||RSX name. (Last character of name has bit 7 set to "1", all other characters have bit 7 set to "0:")statement seperator

Offset Count Description 0 1 *The &1f (note 1) 1 5 Floating point number (note 2) 1. This byte amp;ff code is the BASIC token identifying used as a floating point number 2prefix for more keywords. These 5 bytes define See the floating point numbertable below for the list of keywords using this prefix. * The space symbol is &amp;e2,&amp;e8 and &amp;e9 are not used as a command seperator * Variable definitions: o The variable definition has the following structure: Offset Count Description 0 1 Token defining variable type 2 2 16-bit Byte offset &amp;7c ("|") is a character prefix used to variable value within tokenised BASIC programidentify an RSX command. e.g. "|DIR". 4 x Variable name

The 16-bit byte offset An RSX is initially set to 0, but is setup when the program is RUN. The offset is stored encoded in little-endian format, with low byte followed by high byte. o The token byte defines the variable type (i.e. string, floating point, integer) and the suffix ("$", none and "%"). o The variable name is stored in the program, with bit 7 of the last character set to "1". e.g. The variable named "abc" will be encoded as 'a','b','c'+&80 * String values: o Each string value is prefixed with &22 token. o All strings must be enclosed by double quote symbols ("). This symbols define the start and end of the string value. o The string value can contain any symbol except the quote ("). i.e. 0-&21, and &23-&ff can be used. * After running a program, Tokens of "1d" with 16-bit BASIC program line pointers are changed to token "1e" with using the 16-bit memory address of the BASIC line number in memory. following structure:

This table list the BASIC tokens with a {| border="1"|-! Offset ! Count ! Description|-| 0 | 1 | "&ff #124;" character prefix |-| 1 | 1 | 8-bit byteoffset to tokens following RSX name.|-| 2 | x | RSX name.(Last character of name has bit 7 set to "1", all other characters have bit 7 set to "0")|}

Code*This token identifies a integer (hexidecimal16-bit) BASIC keyword&00 ABS&01 ASC&02 ATN&03 CHR$&04 CINT&05 COS&06 CREAL&07 EXP&08 FIX&09 FRE&0a INKEY&0b INP&0c INT&0d JOY&0e LEN&0f LOG&10 LOG10&11 LOWER$&12 PEEK&13 REMAIN&14 SGN&15 SIN&16 SPACE$&17 SQ&18 SQR&19 STR$number. The two bytes following this token is the number, stored low byte then high byte. e.g. &amp;1a TAN,&1b UNTamp;2d,&1c UPPER$&1d VAL (note 2)&40 EOF&41 ERR&42 HIMEM&43 INKEY$&44 PIamp;00 represents the integer number "&amp;002d"="45 RND&46 TIME&47 XPOS&48 YPOS&49 DERR (v1".1) *This token (note 3&amp;0B)is changed at run-time to &71 BIN$amp;0D &72 DEC$ (v1*This token identifies a integer variable. 02,00,00,var name offset to number in program setup when program RUN. The variable name is stored directly, with bit 7 of the last character set.1)&73 HEX$*This token identifies a string variable. offset to string in program string stored AS IS with quotes around it. &74 INSTR&75 LEFT$&76 MAX&77 MIN&78 POS&79 RIGHT$&7a ROUND&7b STRING$&7c TEST&7d TESTR&7e COPYCHR$ (v1*This token identifies a floating point number. Immediatly following the token are 5 bytes which define the number.1)&7f VPOS (note 4)

* #This byte is the BASIC token identifying a floating point number *#These codes are prefixed by &FF. e.g5 bytes define the floating point number. *The keyword "ABS" space symbol is stored used as a command seperator *Variable definitions: **The variable definition has the following sequencestructure:

* Codes &1e<br> The 16-bit byte offset is initially set to 0, but is setup when the program is RUN.The offset is stored in little-endian format, with low byte followed by high byte..&3f inclusive are not used* Codes &4a...&6f inclusive are not used* Codes &80...&ff inclusive are not used

A floating point number represents *After running a number program, Tokens of "1d" with both an integer and fractional part16-bit BASIC program line pointers are changed to token "1e" with the 16-bit memory address of the BASIC line number in memory.

In Amstrad <br> This table list the BASIC, tokens with a floating point number is stored in base-2 in a normalized form:&amp;ff prefix byte.

1 x 2<exponentbr>

The sign *When a BASIC program is represented by in memory, the BASIC ROM can replace "16-bit 7 of byte 3BASIC program line number" tokens with "16-bit BASIC program line memory address pointer".

The value of the sign When a program is loaded or saved, there will only be "16-bit indicates the sign BASIC program line number" tokens followed by a 16-bit number of the floating point numberline.

* If the sign bit When a program is "1"running, then BASIC will replace the "16-bit BASIC program line number is negative.* If " with the sign "16-bit is "0BASIC program line memory address pointer"token, then and replace the 16-bit line number is positivewith the memory address of the start of that line.

The mantissa holds * When a REM or ' is seen, the normalized numbercharacters following it are output directly (i.e. The exponent is manipulated so that the most significant bit is always 1. Since it is always 1, it is bytes are not stored. Ittokenized) until the end of line or a ':'s value is implied by the representationseen.

It is stored with A floating point number represents a bias of 128number with both an integer and fractional part.

* 128-255 are positive exponents<br> In Amstrad BASIC,* 0a floating point number is stored in base-127 are negative exponents. 2 in a normalized form: 1 x 2^{&lt;exponent&gt;} The representation uses 5 bytes and is stored using the following structure:

The minimum {| border="1"|-! Byte ! colspan="8" | 0 ! colspan="8" | 1 ! colspan="8" | 2 ! colspan="8" | 3 ! colspan="8" | 4|-! Bit ! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0 ! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0 ! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0 ! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0 ! 7 ! 6 ! 5 ! 4 ! 3 ! 2 ! 1 ! 0|-| Function | colspan="8" | (LSB) mantissa (bits 31-24) | colspan="8" | mantissa (bits 23-16) | colspan="8" | mantissa (bits 15-8) | colspan="1" | sign | colspan="7" | (MSB) mantissa (bits 7-0) | colspan="8" | exponent |} === Sign === The sign is represented by bit 7 of byte 3. The value of the sign bit indicates the sign of the floating point number.  #*If the sign bit is "1", then the number is negative. #*If the sign bit is "0 and this describes a ", then the number of 2^-127is positive. === Mantissa === The mantissa holds the normalized number. The exponent is manipulated so that the most significant bit is always 1. Since it is always 1, it is not stored. It's value is implied by the representation. === Exponent ===

The maximum exponent is 255 8-bit and this describes is stored in byte 4. It is stored with a number bias of 2^128.

== BASIC floating#*128-point/real variables ==255 are positive exponents, #*0-127 are negative exponents.

A floating point (real) variable To obtain the signed exponent, you must subtract 128 from the stored exponent. The minimum exponent is 0 and this describes a number with integer of 2^-127. The maximum exponent is 255 and fractional partsthis describes a number of 2^128.

* Numbers are displayed to 9 decimal places.* If the number doesn't have a fractional part (e.g..0) then the fractional part is not displayed.* Trailing zeros (zero digits after the last digit in the fraction) are not displayed* The value is rounded in the following way for display:

a! = 3If the 10th fractional digit is 5 or above, then the number is rounded up.141592654If the 10th fractional digit is 4 or below, then the number is rounded down.e.g. 0.1234567891 is displayed as 0.123456789 0.1234567895 is displayed as 0.12345679

* A real number uses 5 bytes of memory as storage. The format is described above.* The address of a real variable can be found === floating point example in BASIC RAM by:MaV ===

Where "a" should be replaced with That means that you have to turn around the name of first four bytes if you want to read the variablemantissa correctly:2D800000or0010.1101.1000.0000.0000.0000.0000.0000in binary (the stops are inserted for better readability).

A integer variable describes a whole The mantissa has an implied decimal point before the number. Since the number is 2^32 bits (i.e. a number without any fractional part4 bytes) long you have to divide by 2^32 to get the proper decimal representation.

* A integer variable is defined with a "%" character postfix2910846976 / 2^32 = 0.677734375

e.gThe mantissa in a decimal representation is: 0.677734375

0.677734375 * A integer variable has a range of -32768 to 32767* A integer variable always uses two bytes of memory as storage with the following format:2^6 = 43.375

Offset Length Description'''Another example:'''<br>0 2 Integer PI is represented in memory as:<br>'''A2 DA 0F 49 82'''<br>The mantissa is<br>490FDAA2 (highest significant bit is zero => numberis positive)The exponent is<br>82 (=positive mantissa, value=2 thus multiply mantissa with 2^2)<br>

* The address of Delete the integer variable can be found in BASIC bysign bit (which is 0 anyway), and then add the implied 1 bit:<br>C90FDAA2 (hex) = 3373259426 (dec) = 1100.1001.0000.1111.1101.1010.1010.0010 (bin)<br><br>

where "a" should be replaced with Then add the name of the variable.exponent (82h - 80h = 2 thus 2^2)<br>0,7853981633670628070831298828125 * 2^2 = 3,14159265346825122833251953125<br><br>

NOTESIf you compare to a (more) correct representation of PI, you'll see that the CPC is correct up to the 9th decimal place:<br><br>

eCPC:<br>3.14159265346825122833251953125<br>more correctly:<br>3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986..g.

a$ = "hello"= BASIC floating-point/real variables ==

* A string floating point (real) variable is described internally by describes a "string descriptor block" number with integer and fractional parts. The biggest figure which has the following structure:can be stored correctly in RAM by a variable is 2^32-1 = 4294967295.

#*A integer variable is defined with a "!" character postfix. e.g. <pre>a! = 3.141592654</pre> #*A real number uses 5 bytes of memory as storage. The format is described above. #*The address of a real variable can be found in BASIC by: <pre>PRINT @a!</pre> Where "a" should be replaced with the name of the variable.  <br> == BASIC integer variables == A integer variable describes a whole number. i.e. a number without any fractional part.  <br> NOTES:  #*A integer variable is defined with a "%" character postfix. e.g. <pre>a% = 3</pre> #*A integer variable has a range of -32768 to 32767 #*A integer variable always uses two bytes of memory as storage with the following format: {| border="1"|-! Offset ! Length ! Description |-| 0 | 2 | Integer number|} #*The address of the integer variable can be found in BASIC by: <pre>PRINT @a%</pre> where "a" should be replaced with the name of the variable.  == BASIC string variables == A string is a variable which contains a group of characters.  <br> NOTES:  #**A string variable is defined in BASIC with a "$" character postfix. e.g. <pre>a$ = "hello"</pre> #**A string variable is described internally by a "string descriptor block" which has the following structure: {| border="1 "|-! Offset ! Length ! Description|-| 0 | 1 | Length of string in bytes/characters |-| 1 | 2 | Location of start of string|}

#** The address of the "string descriptor block" for the string variable can be found in BASIC by typing:<pre>PRINT @a$</pre>

PRINT @replacing "a$" with the name of the string variable.

replacing "#**A string uses a$minimum of 3 bytes of storage (a 0 character length string) and a maximum of 258 bytes of storage. 3 bytes are used by the " with string descriptor block") #**A string variable can contain any ASCII character code in the name range 0-255. #**A string variable can store a minimum of 0 characters and a maximum of 255 characters. #**The length of the string variablein characters is defined in the string descriptor block. The string does not have a termination character.