Version analysis of BRBR / NEWBRK

This code appears in the following versions (click to see it in the source code):

Disc (docked), 6502 Second Processor and Master

Code variations between these versions are shown below.

Code variation 1 of 10A variation in the comments only

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Docked, 6502SP
Master
       Name: BRBR
       Name: NEWBRK

       Type: Subroutine
   Category: Utility routines
    Summary: The standard BRKV handler for the game

Code variation 2 of 10A variation in the comments only

This variation is blank in the 6502 Second Processor and Master versions.

Docked
  Deep dive: Swapping between the docked and flight code

Code variation 3 of 10A variation in the comments only

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Docked
6502SP, Master
 This routine is used to display error messages. It does this by restarting the
 game to display the title screen, and the TITLE routine then prints the error
 message on-screen.
 This routine is used to display error messages, before restarting the game.
 When called, it makes a beep and prints the system error message in the block
 pointed to by (&FD &FE), which is where the MOS will put any system errors. It
 then waits for a key press and restarts the game.

Code variation 4 of 10A variation in the comments only

This variation is blank in the Master version.

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Docked
6502SP
 BRKV is set to this routine in the loader, when the docked code is loaded, and
 at the end of the SVE routine after the disc access menu has been processed.
 In other words, this is the standard BRKV handler for the game, and it's
 swapped out to MEBRK for disc access operations only.

 When it is the BRKV handler, the routine can be triggered using a BRK
 instruction. The main differences between this routine and the MEBRK handler
 that is used during disc access operations are that this routine restarts the
 game rather than returning to the disc access menu.
 BRKV is set to this routine in the decryption routine at DEEOR just before the
 game is run for the first time, and at the end of the SVE routine after the
 disc access menu has been processed. In other words, this is the standard
 BRKV handler for the game, and it's swapped out to MEBRK for disc access
 operations only.

 When it is the BRKV handler, the routine can be triggered using a BRK
 instruction. The main differences between this routine and the MEBRK handler
 that is used during disc access operations are that this routine restarts the
 game rather than returning to the disc access menu.

Code variation 5 of 10A variation in the labels only

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Docked, 6502SP
Master
.BRBR
.NEWBRK

Code variation 6 of 10Specific to an individual platform

This variation is blank in the Master version.

Docked, 6502SP
                        \ When we call this routine, we know that brkd will be
                        \ zero, as it is initialised to zero and the only other
                        \ place it gets changed is in the TITLE routine, where
                        \ it also gets set to 0

 DEC brkd               \ Set brkd = &FF to indicate that there is a system
                        \ error that needs to be printed out on the title screen
                        \ by the TITLE routine

Code variation 7 of 10Specific to an individual platform

This variation is blank in the Disc (docked) version.

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Master
6502SP
 LDX stackpt            \ Set the stack pointer to the value that we stored in
 TXS                    \ location stack, so that's back to the value it had
                        \ before we change it in the SVE routine

 JSR getzp              \ Call getzp to restore the top part of zero page from
                        \ the buffer at &3000, as this will have been stored in
                        \ the buffer before performing the disc access that gave
                        \ the error we're processsing

 STZ CATF               \ Set the CATF flag to 0, so the TT26 routine reverts to
                        \ standard formatting

 LDY #0                 \ Set Y to 0, which we use as a loop counter below

 LDA #7                 \ Set A = 7 to generate a beep before we print the error
                        \ message

.BRBRLOOP
 LDX #&FF               \ Set the stack pointer to &01FF, which is the standard
 TXS                    \ location for the 6502 stack, so this instruction
                        \ effectively resets the stack

 JSR backtonormal       \ Disable the keyboard and set the SVN flag to 0

 TAY                    \ The call to backtonormal sets A to 0, so this sets Y
                        \ to 0, which we use as a loop counter below

 LDA #7                 \ Set A = 7 to generate a beep before we print the error
                        \ message

.BRBRLOOP

Code variation 8 of 10A variation in the labels only

This variation is blank in the Disc (docked) version.

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Master
6502SP
 JSR CHPR               \ Print the character in A, which contains a line feed
                        \ on the first loop iteration, and then any non-zero
                        \ characters we fetch from the error message
 JSR OSWRCH             \ Print the character in A, which contains a line feed
                        \ on the first loop iteration, and then any non-zero
                        \ characters we fetch from the error message

Code variation 9 of 10Specific to an individual platform

This variation is blank in the Disc (docked) version.

6502SP, Master
 INY                    \ Increment the loop counter

 LDA (&FD),Y            \ Fetch the Y-th byte of the block pointed to by
                        \ (&FD &FE), so that's the Y-th character of the message
                        \ pointed to by the MOS error message pointer

 BNE BRBRLOOP           \ If the fetched character is non-zero, loop back to the
                        \ JSR OSWRCH above to print the it, and keep looping
                        \ until we fetch a zero (which marks the end of the
                        \ message)

Code variation 10 of 10Minor and very low-impact

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Docked
Master
6502SP
 BNE BR1                \ If brkd is non-zero then it must be &FF, which
                        \ indicates that where is a system error that we need to
                        \ print, so jump to BR1 to restart the game and fall
                        \ through into the TITLE routine to print the error
 JSR t                  \ Scan the keyboard until a key is pressed, returning
                        \ the ASCII code in A and X

 JMP SVE                \ Jump to SVE to display the disc access menu and return
                        \ from the subroutine using a tail call
 JMP BR1                \ Jump to BR1 to restart the game