BBC BASIC Programmers' Reference

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=====Reading and writing nonstandard values=====

//by J.G.Harston, July 2009//\\  Read a 16-bit integer\\ 
<code bb4w>
        DEF FNrd16(I%):=BGET#I%+256*BGET#I%
</code>
Read a 32-bit integer (same as **INPUT#I%,A%** but as a function, see [[/Reading%20and%20writing%20nonstandard%20values#notes1|notes]])\\ 
<code bb4w>
        DEF FNrd32(I%):LOCAL A%:DIM zp% LOCAL 3:FOR A%=0 TO 3:zp%?A%=BGET#I%:NEXT:=!A%
</code>
Read a length-specified string (string length followed by string of up to 255 characters)\\ 
<code bb4w>
        DEF FNrdStrN(I%):LOCAL A%,A$:A%=BGET#I%:IFA%=0:=""
        REPEATA$=A$+BGET#I%:A%=A%-1:UNTILA%<1:=A$
</code>
Read a length-specified reversed string (string length followed by reversed string of up to 255 characters)\\ 
<code bb4w>
        DEF FNrdStrR(I%):LOCAL A%,A$:A%=BGET#I%:IFA%=0:=""
        REPEATA$=BGET#I%+A$:A%=A%-1:UNTILA%<1:=A$
</code>
Read a bit7-terminated string (final character has bit 7 set)\\ 
<code bb4w>
        DEF FNrdStrB7(I%):LOCAL A%,A$:REPEAT:A%=BGET#I%:A$=A$+CHR$(A%AND127):UNTILA%>127:=A$
</code>
Write a 16-bit integer\\ 
<code bb4w>
        DEF PROCwr16(O%,A%):DIM zp% LOCAL 3:!zp%=A%:FOR A%=0 TO 1:BPUT#O%,zp%?A%:NEXT:ENDPROC
</code>
Write a 32-bit integer (same as **PRINT#O%,A%**, see [[/Reading%20and%20writing%20nonstandard%20values#notes1|notes]])\\ 
<code bb4w>
        DEF PROCwr32(O%,A%):DIM zp% LOCAL 3:!zp%=A%:FOR A%=0 TO 3:BPUT#O%,zp%?A%:NEXT:ENDPROC
</code>
Write a length-specified string (string length followed by string of up to 255 characters)\\ 
<code bb4w>
        DEF PROCwrStrN(O%,A$):LOCAL A%:BPUT#O%,LENA$:IFA$="":ENDPROC
        FOR A%=1 TO LEN A$:BPUT#O%,ASCMID$(A$,A%,1):NEXT:ENDPROC
</code>
Write a length-specified reversed string (string length followed by reversed string of up to 255 characters)\\ 
<code bb4w>
        DEF PROCwrStrN(O%,A$):LOCAL A%:BPUT#O%,LENA$:IFA$="":ENDPROC
        FOR A%=LEN A$ TO 1 STEP -1:BPUT#O%,ASCMID$(A$,A%,1):NEXT:ENDPROC
</code>
Write a bit7-terminated string (final character has bit 7 set)\\ 
<code bb4w>
        DEF PROCwrStrB7(O%,A$):BPUT#O%,LEFT$(A$,LENA$-1);:BPUT#O%,ASCRIGHT$(A$,1)OR128:ENDPROC
</code>
===== Notes =====
Different varieties of BBC BASIC write and read data with **PRINT#** and **INPUT#** in different ways. You can only be certain that **INPUT#** will read data written by **PRINT#** with the same BBC BASIC. These functions allow you to impose the exact structure on the data you are writing, regardless of what platform BBC BASIC the program is running or where the data is going to or coming from.\\ \\ 
==== Comments by Richard Russell ====
 I don't believe that **INPUT#I%,A%** reads four bytes, nor that **PRINT#O%,A%** writes four bytes, in any version of BBC BASIC. In //BBC BASIC for Windows// they read/write **five** bytes (in ***FLOAT40** mode) or **eight** bytes (in ***FLOAT64** mode).\\ \\ **FNrd32** is more straightforwardly implemented as follows (it should also be considerably faster than the code listed above):\\ 
<code bb4w>
        DEF FNrd32(I%)=BGET#I%+(BGET#I%<<8)+(BGET#I%<<16)+(BGET#I%<<24)
</code>
The 'length-specified' string routines appear to assume a maximum string length of 255 characters, but this is not stated.\\ \\  In several instances the routines listed could be simplified at the expense of compatibility with old versions of BBC BASIC (e.g. 6502 and Z80), for example by using the **+=** and **–=** operators.