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[[Category:Cpctech.org]]
'''''This artikel article originally came from Kevin Thackers' archive at [http://www.cpctech.org.uk http://www.cpctech.org.uk].'''''
= Technical information about Locomotive BASIC =
A RSX (Resident System Extension) command is accessed through BASIC with a "|" character prefix. e.g. The "DIR" RSX, is accessed by "|DIR". A binary function is accessed through BASIC using the "CALL" command.
Both RSX and CALL commands works work (are!) similar from the BASIC command line and invokes a invoke machine code - the only difference is: 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:
Passing for integer:
<pre>
...
MSB PARAMn
IX+2 0 ------> LSB PARAMn
</pre>
with the help of that assembly code is it possible to find the parameters:
ROUT1: CP "x" ;test register A if "x" parameters were hand-overed, e.g. "x" = 2
RET NZ ; if no > failure and return.
LD L,(IX+20) LD H,(IX+31) ; HL = value of last parameter LD E,(IX+42) LD D,(IX+53) ; 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, the 16-bit value is an address of a "string descriptor block". The first byte of the string description or 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 param 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.
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 programm program line number)
: - no space (or 'under' as it is common today) between variable names are allowed
: - BASIC keywords can't be used
