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by alphaBetaGamma
2232 days ago
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If I remember correctly, the system calls where indirect jumps where the address of the jump was in ram (it had been copied from the rom on startup): it was designed to you could hijack system calls and modify their behavior. This blew my mind as a teenager. I wish modern system where still designed like that... |
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There were a handful of pointers to the routines to use for BASIC execution in RAM starting at address $300, which could be pointed elsewhere for BASIC extenders or to redirect IRQs. Apart from that, most syscall addresses where in ROM.
It allowed disabling parts of the ROM though to access the full RAM. That way you could rewrite the real jump addresses, bypassing the ROM completely.