From BeebWiki
Revision as of 21:51, 7 September 2021 by Jgharston (talk | contribs) (Specification)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

Initialize to read a string


 6502  On entry: On exit:
Cy CC = Space or CR terminates unquoted string  
CS = CR terminates unquoted string
Z undefined NE = String present
EQ = No string, end of line
A undefined Opening character of string;
&0D if no string present, end of line
X undefined preserved
Y Offset of first character to scan Offset of first character of string
(&F2),Y   Points to start of string (&F2) preserved, Y updated

Prepares GSREAD to read a string from a character array. 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.

GSINIT starts at the position given by (&F2),Y and scans forward, skipping leading spaces.

If the string begins with " then &22 is returned in A and (&F2),Y points to the character after the ". In this case the Carry flag input to GSINIT has no effect, GSREAD will return characters until the closing " is reached. A CR encountered before the closing " causes error &FD, Bad string.

Otherwise (&F2),Y points to the first non-space character, which is returned in A. The Zero flag returns EQ if this is CR, indicating an empty, unquoted string. GSREAD will return characters until CR is reached, or also space if GSINIT was called with Carry Clear.

While processing a command line, the Zero flag set to NE on exit from GSINIT indicates that an argument is present. GSINIT returning EQ is distinct from the first call to GSREAD returning CS which indicates a null string, not an absent string.

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.


The following examples should make the CLC/SEC entry options clearer. In each case it shows where the string is terminated indicated with a | character.

CLC on entry

  • some| words
  • alongstringofwords|
  • "some words"| and more words
  • some"words| and" more words

This is *LOAD filename style parsing.

SEC on entry

  • some words|
  • alongstringofwords|
  • "some words"| and more words
  • some"words and" more words|

This is *KEY style parsing.


GSINIT modifies GSFLAG in location &E4, a zero page location also used while processing numerical *commands (*FX, *OPT, *TV etc.) The value on exit is used by subsequent GSREAD calls, and is made up as follows:

Bit Meaning
7 " found at start of string
6 CS on entry; only CR terminates unquoted string
5..0 Unused; bits 7..2 of previous value

Entry points

  • 6502 Entry Address: &FFC2 (I/O processor only)

Regregex (talk) 17:42, 18 August 2021 (CEST)