Difference between revisions of "GSINIT"
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+ | Initialize to read a string | ||
+ | |||
+ | ==Specification== | ||
+ | {| cellpadding="0" cellspacing="0" | ||
+ | | 6502 || align="left" | '''On entry:''' || align="left" | '''On exit:''' | ||
+ | |- align="center" valign="top" | ||
+ | | Z || align="left" | undefined || align="left" | 0=String is non-empty or quoted<br>1=String is empty and unquoted | ||
+ | |- align="center" valign="top" | ||
+ | | Cy || align="left" | 0=Space, CR or closing <tt>"</tt> terminates string<br>1=CR or closing <tt>"</tt"> terminates string || align="left" | undefined | ||
+ | |- align="center" valign="top" | ||
+ | | A || align="left" | undefined || align="left" | First character of string; =&0D if string empty and unquoted | ||
+ | |- align="center" valign="top" | ||
+ | | X || align="left" | undefined || align="left" | preserved | ||
+ | |- align="center" valign="top" | ||
+ | | Y || align="left" | Offset of first character to scan || align="left" | Offset of first character of string | ||
+ | |- align="center" valign="top" | ||
+ | | ?&F2 || align="left" | LSB of start of array || align="left" | preserved | ||
+ | |- align="center" valign="top" | ||
+ | | ?&F3 || align="left" | MSB of start of array || align="left" | preserved | ||
+ | |} | ||
+ | |||
+ | Prepares [[GSREAD]] to read a string from a character array. The string may be enclosed in double quotes (<tt>"</tt>) 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,?&F3)+Y and scans forward, incrementing Y until a character other than space or [[VDU 13|CR]] is found. | ||
+ | |||
+ | If the string begins with <tt>"</tt> then &22 is returned in A and (?&F2,?&F3)+Y points to the character after the <tt>"</tt>. In this case the carry flag input to GSINIT has no effect; GSREAD will return characters until the closing <tt>"</tt> is reached. CR encountered before the closing <tt>"</tt> causes error &FD, '''Bad string'''. | ||
+ | |||
+ | Otherwise (?&F2,?&F3)+Y points to the first non-space character, which is returned in A. Z is set 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 Cy=0. | ||
+ | |||
+ | It is safe to call GSREAD at least once following GSINIT, and repeatedly on reaching CR. | ||
+ | |||
+ | ;Note | ||
+ | The character returned by GSINIT is ''not'' necessarily the character returned by the first call to [[GSREAD]]. | ||
+ | |||
+ | ==Calling from BBC BASIC== | ||
+ | |||
+ | ==Entry points== | ||
+ | * 6502 Entry Address: &FFC2 (I/O processor only) | ||
+ | |||
+ | [[User:Regregex|Regregex]] ([[User talk:Regregex|talk]]) 17:53, 11 January 2019 (CET) |
Revision as of 14:21, 25 July 2019
Initialize to read a string
Specification
6502 | On entry: | On exit: |
Z | undefined | 0=String is non-empty or quoted 1=String is empty and unquoted |
Cy | 0=Space, CR or closing " terminates string 1=CR or closing "</tt"> terminates string |
undefined |
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 | LSB of start of array | preserved |
?&F3 | MSB of start of array | preserved |
Prepares GSREAD to read a string from a character array. The string may be enclosed in double quotes (<tt>") 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,?&F3)+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,?&F3)+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. CR encountered before the closing " causes error &FD, Bad string.
Otherwise (?&F2,?&F3)+Y points to the first non-space character, which is returned in A. Z is set 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 Cy=0.
It is safe to call GSREAD at least once following GSINIT, and repeatedly on reaching CR.
- Note
The character returned by GSINIT is not necessarily the character returned by the first call to GSREAD.
Calling from BBC BASIC
Entry points
- 6502 Entry Address: &FFC2 (I/O processor only)