introduction_to_programming_the_beamracer
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introduction_to_programming_the_beamracer [2020/10/08 14:45] – [Local Memory] laubzega | introduction_to_programming_the_beamracer [2021/09/10 01:49] (current) – [Initialization] silverdr | ||
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In order to maintain a high level of compatibility with existing C64 software, BeamRacer remains hidden on power up, and the computer behaves as if it was not there. That is, register writes are ignored, and reads report the same values as those in a vanilla C64. | In order to maintain a high level of compatibility with existing C64 software, BeamRacer remains hidden on power up, and the computer behaves as if it was not there. That is, register writes are ignored, and reads report the same values as those in a vanilla C64. | ||
- | It is necessary to perform so-called “register knocking” for the board to reveal its presence. This is achieved by writing a sequence $42, $52 (screen codes for “BR”) to register $D031. To check if the board is indeed there, register $D031 can then be read and compared with “0”. | + | It is necessary to perform so-called “register knocking” for the board to reveal its presence. This is achieved by writing a sequence $42, $52 (screen codes for “BR”) to register $D031. To check if the board is indeed there, register $D031 can then be read and verified. Value of $FF means that BeamRacer was NOT activated.((Please note that early revisions of this chapter, and the code snippet below, advocated checking for $00 as an indicator that the BeamRacer was activated. This is now deprecated and may lead to "false negative" |
< | < | ||
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STX VREG_CONTROL | STX VREG_CONTROL | ||
LDX VREG_CONTROL | LDX VREG_CONTROL | ||
- | BEQ BEAMRACER_FOUND_AND_ACTIVATED | + | INX |
+ | BNE BEAMRACER_FOUND_AND_ACTIVATED | ||
RTS ; sadly, no BeamRacer... | RTS ; sadly, no BeamRacer... | ||
</ | </ |
introduction_to_programming_the_beamracer.1602193532.txt.gz · Last modified: 2020/10/08 14:45 by laubzega