BBC BASIC Programmers' Reference

by Richard Russell, January 2015

You may wish to convert a number to a string in assembly language (equivalent to the STR$ function in BASIC). This can easily be done by calling an internal routine within the interpreter. Here is how to do it (requires BBC BASIC for Windows version 6):

      ; eax = print format control (equivalent to @%)
      ; bx|ecx|edx = number to print (80-bit variant)
      ; edi = address of destination buffer
      push eax
      mov  ebp,esp
      call @fn%(12)
      pop  eax
      mov  byte [edx+ecx],0

On completion a NUL-terminated string is stored in the supplied buffer.

To illustrate the use of this code here is a complete program which prints the value of PI using assembly language:

      DIM Buffer% 255, P% 100, L% -1
      [OPT 10
      .ppi
      mov  eax,&1414     ; G20 format
      mov  bx,&4000      ; Value of PI ...
      mov  ecx,&C90FDAA2 ; .. as an ...
      mov  edx,&2168C235 ; .. 80-bit float
      mov  edi,Buffer%   ; Destination buffer

      push eax
      mov  ebp,esp
      call @fn%(12)
      pop  eax
      mov  byte [edx+ecx],0

      mov  esi,Buffer%
      .vout
      lodsb
      call "oswrch"
      or   al,al
      jnz  vout
      ret
      ]

      CALL ppi
      PRINT