mirror of https://github.com/mgcaret/rom4x.git
22 lines
710 B
ArmAsm
22 lines
710 B
ArmAsm
; Hijack the BELL1 monitor routine to do our bidding.
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; BELL1 implements the beep sound heard on reset or
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; Ctrl-G, etc. It starts with
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; LDA #$40
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; JSR WAIT ; delay .1 sec
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; followed by code to actually beep the speaker
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; In our case, BELL1 always loads the accumulator with
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; a fixed number, and executes a 3-byte instruction
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; Well, it turns out that to switch banks we need
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; 3 bytes, and as luck would have it the other bank
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; is empty here.
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; So the routine on the other side is the ROM 5X
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; dispatcher. It will take what is in the accumulator
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; and use that to determine the next action.
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; Obviously, $40 should beep the speaker, anything
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; else can do whatever we want.
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.code
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.org $fbdf
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sta $c028
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