     $DF21/57121:   Write side-sector

    DF21: 85 F9     STA $F9         ; buffer number
    DF23: A9 90     LDA #$90        ; command code for writing

    Jump from $DF1F:

    DF25: 48        PHA
    DF26: B5 EC     LDA $EC,X
    DF28: 29 01     AND #$01        ; isolate drive number
    DF2A: 85 7F     STA $7F
    DF2C: 68        PLA
    DF2D: 05 7F     ORA $7F         ; command code plus drive number
    DF2F: 8D 4D 02  STA $024D       ; save
    DF32: B1 94     LDA ($94),Y     ; track number
    DF34: 85 80     STA $80
    DF36: C8        INY
    DF37: B1 94     LDA ($94),Y     ; sector number
    DF39: 85 81     STA $81
    DF3B: A5 F9     LDA $F9         ; buffer number
    DF3D: 20 D3 D6  JSR $D6D3       ; transmit parameter to disk controller
    DF40: A6 F9     LDX $F9         ; buffer number
    DF42: 4C 93 D5  JMP $D593       ; transmit command to disk controller

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