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Routines |
| Prev: $3687 | Up: Map | Next: $3724 |
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| $36E3 | LDA #$08 | Prepare to address the screen refresh buffer (SRB) at $4008. | ||||||||
| $36E5 | STA $52 | |||||||||
| $36E7 | LDA #$40 | |||||||||
| $36E9 | STA $53 | |||||||||
| $36EB | LDA #$00 | Calculate the index of the SRB byte that corresponds to the leftmost segment of 8 character cells in the row at the given y-coordinate and store it at $55. | ||||||||
| $36ED | LDY $4A | |||||||||
| $36EF | BEQ $36F7 | |||||||||
| $36F1 | CLC | |||||||||
| $36F2 | ADC #$05 | |||||||||
| $36F4 | DEY | |||||||||
| $36F5 | BNE $36F1 | |||||||||
| $36F7 | STA $55 | |||||||||
| $36F9 | LDA $49 | Pick up the x-coordinate from $49, divide by 8 and store the result at $56. This will be added to the index at $55 later to get the index of the target SRB byte. | ||||||||
| $36FB | LSR A | |||||||||
| $36FC | LSR A | |||||||||
| $36FD | LSR A | |||||||||
| $36FE | STA $56 | |||||||||
| $3700 | ASL A | Store the multiple of 8 closest to but not greater than the x-coordinate at $54. | ||||||||
| $3701 | ASL A | |||||||||
| $3702 | ASL A | |||||||||
| $3703 | STA $54 | |||||||||
| $3705 | LDA $49 | Compute the remainder when the x-coordinate is divided by 8 and transfer it to Y. This indicates the bit of the SRB byte that needs to be set. | ||||||||
| $3707 | SEC | |||||||||
| $3708 | SBC $54 | |||||||||
| $370A | TAY | |||||||||
| $370B | LDA #$80 | Set the bit in A that needs to be set in the SRB byte. | ||||||||
| $370D | CPY #$00 | |||||||||
| $370F | BEQ $3715 | |||||||||
| $3711 | LSR A | |||||||||
| $3712 | DEY | |||||||||
| $3713 | BNE $3711 | |||||||||
| $3715 | STA $54 | Store A (with the appropriate bit set) at $54. | ||||||||
| $3717 | LDA $55 | Compute the index of the SRB byte that needs updating and transfer it to Y. | ||||||||
| $3719 | CLC | |||||||||
| $371A | ADC $56 | |||||||||
| $371C | TAY | |||||||||
| $371D | LDA ($52),Y | Set the appropriate bit in the SRB byte. | ||||||||
| $371F | ORA $54 | |||||||||
| $3721 | STA ($52),Y | |||||||||
| $3723 | RTS | |||||||||
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