Hmm.. struggling a bit with OSWORD=&00 - probably because I've never used the BRK instruction before.
![Image]()
If Escape is pressed, there's a JMP $9838
9838 just seems to contain a single BRK instruction.. so wouldn't that cause the execution to occur from the vector at FFFE?
Looking at FFFE, we have address DC1C.So it must branch into $DC27 below, right?
So ultimately, using vector $0202, control is handed back to $b402, correct? I'm hoping I can just skip the entire code from MOS and continue execution at $b402.
![Image]()
If Escape is pressed, there's a JMP $9838
9838 just seems to contain a single BRK instruction.. so wouldn't that cause the execution to occur from the vector at FFFE?
Looking at FFFE, we have address DC1C.
Code:
This page ([url]https://mdfs.net/Docs/Comp/BBC/OS1-20/DC1C[/url]) tells me it calls this code:DC1CSTA &FC ;save ADC1EPLA ;get back status (flags)DC1FPHA ;and save againDC20AND #&10 ;check if BRK flag setDC22BNE &DC27 ;if so goto DC27DC24JMP (&0204) ;else JMP (IRQ1V)Code:
************************************************************************** ** BRK handling routine ** **************************************************************************DC27TXA ;save X on stackDC28PHA ;DC29TSX ;get status pointerDC2ALDA &0103,X ;get Program Counter loDC2DCLD ;DC2ESEC ;set carryDC2FSBC #&01 ;subtract 2 (1+carry)DC31STA &FD ;and store it in &FDDC33LDA &0104,X ;get hi byteDC36SBC #&00 ;subtract 1 if necessaryDC38STA &FE ;and store in &FEDC3ALDA &F4 ;get currently active ROMDC3CSTA &024A ;and store it in &24ADC3FSTX &F0 ;store stack pointer in &F0DC41LDX #&06 ;and issue ROM service call 6DC43JSR &F168 ;(User BRK) to ROMs ;at this point &FD/E point to byte after BRK ;ROMS may use BRK for their own purposesDC46LDX &028C ;get current languageDC49JSR &DC16 ;select ROMDC4CPLA ;get back original value of XDC4DTAX ;DC4ELDA &FC ;get back original value of ADC50CLI ;allow interruptsDC51JMP (&0202) ;and JUMP via BRKV (normally into current language)
Statistics: Posted by SparkyNZ — Sun Feb 18, 2024 6:01 am