The BBC and Master Computer Public Domain Library

B32B Set Numeric Variable

Submitted by Steve Fewell

Routine:setnumericvar
Name: Store numerical value in the specified variable
Starting Address: &B32B
Entry criteria: Either the IWA or the FWA
contains the value & the variable address and type details are stored on the Stack (3 bytes).
Exit: The specified variable has been set to the numerical value from the IWA or FWA.

Description:

(if called from &B328 then evaluate the expression value (this will be the value that we
will set the variable to).

Temporarily pop the return address from the Stack (that is the address to return to after the end
of subroutine RTS statement!).
Pop the variable return type from the stack and store this value in &39.
Pop the Variable value address from the stack and store this address in &37-&38.
Push the return address back onto the stack.
If called from &B338, then the stack manipulation is skipped, as the variable address details and variable type
information has already been set (and ae not located on the stack).

If the variable return type is 5 then the variable requires a Floating-Point value, so do the following:

* If the value type (for the current value in the IWA/FWA, in &27) is zero then we have
   a string value, so issue a Type Mismatch error, as we cannot assign a String to our variable.
* If the value type is positive then we have an Integer value that we need to assign to a Floating-Point
   variable, so convert the Integer to a Floating-Point value.
* Store the FWA to the variable's Memory address (&B369) to output the FWA value
   to the address pointed to by &37-&38, and then exit.
   Note: the FWA will be stored in it's 5-byte packed format, as follows:
      Byte 1: The FWA Exponent
      Byte 2: The FWA Sign Bit (MSB) followed by the lower 7-bits of Mantissa Byte 1
         (as the FWA value is normalised, we don't need to store the TOP bit of
          the FWA Mantissa byte 1 value (as it is always 1), so the FWA sign
          value replaces this bit).
      Byte 3: The FWA Mantissa Byte 2
      Byte 4: The FWA Mantissa Byte 3
      Byte 5: The FWA Mantissa Byte 4

Otherwise, the variable return type is Integer, so do the following:

* If the value type (for the current value in the IWA/FWA, in &27) is zero then we have
   a string value, so issue a Type Mismatch error, as we cannot assign a String to our variable.
* If the value type is negative then we have a Floating-Point value that we need to assign to an Integer
   variable, so convert the Floating-Point value to an Integer.
* Continue to the Store IWA to Memory address routine (&B347) to output the IWA value
   to the address pointed to by &37-&38, and exit.
   Note The Store IWA to address routine will check the variable type (&39)
   to see if we need to store a 1-byte or 4-byte value, as if the variable is a direct
   memory access (e.g. ?&70 = 1), then we must only store 1-byte; all other Integer values
   are 4-byte variables.

Disassembly for the Set Numerical variable routine