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Read character from string


 6502  On entry: On exit:
Cy undefined CC = Character valid
  VS = control character, A<&20
  VC = not control character, A>&1F
CS = End of string reached
  EQ = end of line
  NE = not end of line, more text on line
A undefined Current character of string, also =&0D if end of string reached
X undefined preserved
Y Offset of current character   Offset of next character
(&F2),Y the string being parsed (&F2) preserved

Returns one character at a time from a string found using GSINIT. The string may be enclosed in double quotes (") in which case spaces in the string are preserved. The string may also contain control codes and high-order characters encoded with an escape sequence.

GSREAD parses the character sequence at the position given by (&F2),Y and scans forward.

If the string began with " then a character is returned with Carry Clear unless the closing " has been reached, then Carry Set is returned with A=&0D. CR encountered before the closing " causes error &FD, Bad string.

Otherwise a character is returned with Carry Clear unless it is a CR, or a space if GSINIT was called with Carry Clear, then Carry Set is returned with A=&0D.

(&F2),Y is left pointing to the next character unless Carry Set is returned, in which case GSREAD skips any spaces following the end of the string.

While processing a command line, GSINIT returning Not Equal indicates that an argument is present. When the first call to GSREAD returns Carry Set, the flag from GSINIT distinguishes between an empty argument and no argument.

It is safe to call GSREAD at least once following GSINIT. However, after GSREAD returns Carry Set it must not be called again without an intervening GSINIT, otherwise, overshooting a closing " and reaching CR also raises a Bad string error.


The character returned by GSINIT is not necessarily the character returned by the first call to GSREAD.


 \ Testing for no string present at all:
 BEQ errNoString
 \ Testing for null string:
 BCS nullstring
 \ Stepping past a string (eg to step past a filename to following parameters):
 BCC loop
 \ Y now points to any parameter after the string
 \ Copying a string
 LDX #0
 STA buffer,X
 INX:BNE loop
 \ X=length of string parsed
 \ Y=updated to parse another parameter, if present


GSREAD tests the top two bits of GSFLAG in location &E4, a zero page location set up by GSINIT but also modified while processing numerical *commands (*FX, *OPT, *TV etc.) The value is preserved during the call, and is made up as follows:

Bit Meaning
7 " found at start of string
6 CS on entry to GSINIT; only CR terminates unquoted string
5..0 Unused

GSREAD also updates GSCHAR in location &E5, clearing or setting bit seven to process the |! sequence.

Entry points

  • 6502 Entry Address: &FFC5 (I/O processor only)

Regregex (talk) 17:48, 18 August 2021 (CEST)