Help with 2-byte indexing lookup table

Hello,

I am going through some code from the "Nerdy Nights" tutorial. In it there is a piece of code that I am having trouble understanding, which uses a 2 byte lookup table for indexing.

Here is part of the lookup table and the code;So, we need to access two bytes from the lookup table to store into these registers ($4002 & $4003). to play a musical note.

In the code above, why can't we just load the y register with the value #$18 (24 decimal), rather than;
lda #$0C ;load a with 12
asl a ;shift left to multiple by 2, now A contains 24 dec (#$18)
tay
lda note_table, y ;read the low byte of the period, y = 24 and loads the value $03 from the note table above
sta $4002 ;write to SQ1_LO, write $03 to $4002
lda note_table+1, y ;read the high byte of the period
sta $4003 ;write to SQ1_HI, write second byte $f8 (from note table) to $4003

Thanks for any help anyone might be able to offer.
TonyAme

Statistics: Posted by trony — Mon Mar 25, 2024 11:29 pm — Replies 1 — Views 34

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