Difference between revisions of "GSINIT"

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(Added examples.)
m (SEC on entry)
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===SEC on entry===
===SEC on entry===
* some words|
* some words|
* alongstringofwords
* alongstringofwords|
* "some words"| and more words
* "some words"| and more words
* some"words and" more words|
* some"words and" more words|

Revision as of 15:13, 24 August 2021

Initialize to read a string


 6502  On entry: On exit:
Z undefined NE = String is non-empty or quoted
EQ = String is empty and unquoted
Cy CC = Space or CR terminates unquoted string  
CS = CR terminates unquoted string
A undefined First character of string; =&0D if string empty and unquoted
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, incrementing Y until a character other than space or CR is found.

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. 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.


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)