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Added more details about how the CBM exec() works.

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This commit is contained in:
Greg King 2013-09-04 09:16:00 -04:00
parent 3969d920fa
commit 875212a850
1 changed files with 30 additions and 11 deletions
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41
libsrc/cbm/exec.c
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@ -1,12 +1,18 @@
/*
** Program-chaining function for Commodore platforms.
**
** 2013-08-24, Greg King
** 2013-09-04, Greg King
**
** This function exploits the program-chaining feature in CBM BASIC's ROM.
** It puts the desired program's name and unit number into a LOAD statement.
** Then, it points BASIC to that statement, so that the ROM will run that
** statement after this program quits. The ROM will load the next program,
**
** CC65's CBM programs have a BASIC program stub. We start those programs by
** RUNning that stub; it SYSes to the Machine Language code. Normally, after
** the ML code exits, the BASIC ROM continues running the stub. But, it has
** no more statements; so, the program stops.
**
** This function puts the desired program's name and device number into a LOAD
** statement. Then, it points BASIC to that statement, so that the ROM will run
** that statement after this program quits. The ROM will load the next program,
** and will execute it (because the LOAD will be seen in a running program).
*/
@ -23,6 +29,24 @@
#endif
/* The struct below is a line of BASIC code. It sits in the LOWCODE segment
** to make sure that it won't be hidden by a ROM when BASIC is re-enabled.
** The line is:
** 0 LOAD""+"" ,01
** After this function has written into the line, it might look like this:
** 0 LOAD""+"program name" ,08
**
** When BASIC's LOAD command asks the Kernal to load a file, it gives the
** Kernal a pointer to a file-name string. CC65's CBM programs use that
** pointer to give a copy of the program's name to main()'s argv[0] parameter.
** But, when BASIC uses a string literal that's in a program, it points
** directly to that literal -- in the models that don't use banked RAM
** (Pet/CBM, VIC-20, and 64). The literal is overwritten by the next program
** that's loaded. So, argv[0] would point to machine code. String operations
** create a new result string -- even when that operation changes nothing. The
** result is put in the string space at the top of BASIC's memory. So, the ""+
** in this BASIC line guarantees that argv[0] will get a name from a safe place.
*/
#pragma data-name(push, "LOWCODE")
static struct line {
const char end_of_line;
@ -33,13 +57,8 @@ static struct line {
const char comma;
char unit[3];
} basic = {
'\0', &basic + 1, /* high byte must be non-zero */
0, 0x93,
/* This string operation copies the name to high BASIC RAM.
** So, it won't be overwritten when the next program is loaded.
*/
"\"\"", 0xaa, '\"',
'\0', &basic + 1, /* high byte of link must be non-zero */
0, 0x93, "\"\"", 0xaa, '\"',
"\" ", /* format: "123:1234567890123456\"" */
',', "01"
};