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Elite on the BBC Micro

Version analysis of MEBRK

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

Code variations between these versions are shown below.

Name: MEBRK Type: Subroutine Category: Save and load Summary: The BRKV handler for disc access operations
This routine is used to display error messages from the disc filing system while disc access operations are being performed. When called, it makes a beep and prints the system error message in the block pointed to by (&FD &FE), which is where the disc filing system will put any disc errors (such as "File not found", "Disc error" and so on). It then waits for a key press and returns to the disc access menu. BRKV is set to this routine at the start of the SVE routine, just before the disc access menu is shown, and it reverts to BRBR at the end of the SVE routine after the disc access menu has been processed. In other words, BRBR 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 difference between this routine and the standard BRKV handler in BRBR is that this routine returns to the disc access menu rather than restarting the game, and it doesn't decrement the brkd counter.
.MEBRK LDX stack \ Set the stack pointer to the value that we stored in TXS \ the stack variable, so that's back to the value it had \ before we set BRKV to point to MEBRK in the SVE \ routine

Code variation 1 of 2Related to Elite's use of the Tube

Tap on a version to expand it, and tap it again to show to all variations.

Docked

6502SP

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

.MEBRKL

 JSR OSWRCH             \ Print the character in A (which contains a beep on the
                        \ first loop iteration), and then any non-zero
                        \ characters we fetch from the error message

 INY                    \ Increment the loop counter

Code variation 2 of 2Other (e.g. bug fix, optimisation)

This might be a bug fix? The 6502SP version returns from the disc access break handler if the error printing routine loops through a whole page, so it always exits. The other versions could crash if there is no null value in the block pointed to by (&FD &FE), so perhaps this fixes this issue?

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

6502SP

BEQ retry \ If Y = 0 then we have worked our way through a whole \ page, so jump to retry to wait for a key press and \ display the disc access menu (this BEQ is effectively \ a JMP, as we didn't take the BNE branch above)
 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 MEBRKL             \ 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)

 BEQ retry              \ Jump to retry to wait for a key press and display the
                        \ disc access menu (this BEQ is effectively a JMP, as we
                        \ didn't take the BNE branch above)

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