Another nametable compression format

    000nnnnn ...    : 32-N literal bytes.
    00111111        : End stream.
    00111110 xx yy  : * Set PPU_ADDR to yyxx
    001nnnnn xx yy  : For 32-N times, emit alternately X and Y.
    01011111        : * toggle BG transperency. (initially opaque)
    010nnnnn xx     : 32-N run of X.
    01111111 xx     : Set BG to X. (initially BG = 0)
    011nnnnn xx     : 32-N incrementing run starting at X.
    1nnnnnnn        : 128-N Run of BG.
    * = Not present in lite variant.

; Copyright (C) 1998-2015 Jeremy Chadwick. All rights reserved.
;
; Redistribution and use in source and binary forms, with or without
; modification, are permitted provided that the following conditions
; are met:
;
; 1. Redistributions of source code must retain the above copyright
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; 2. Redistributions in binary form must reproduce the above copyright
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;
; THIS SOFTWARE IS PROVIDED BY AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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; ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
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; SUCH DAMAGE.

; This is an RLE decompression routine for CHR data.  LOGO_ADDR (a
; 16-bit pointer) is used to reference the base address of the data in
; question.  The format of the data is simple:
;
; Offset 0: Count: Number of times Value should be repeated.
;           If the MSB is set, indicates this is the last entry to be
;           handled.  Number of times is therefore limited to $00-7F
; Offset 1: Value: Raw 8-bit value (i.e. tile number)
; ...repeat...
;
; An example set of compressed data would be:
;
;   .byte $10, $32   ; Repeat sixteen ($10) times the value $32
;   .byte $06, $FA   ; Repeat six ($06) times the value $FA
;   .byte $9F, $1D   ; Repeat thirty-one ($1F) times the value $1D, then done
;
.proc Load_RLE_Data
L1: ldy #0
    lda (LO_ADDR),y          ; Get Count from data structure
    pha                      ; ...and temporarily back it up for later use
    and #$7f                 ; Strip off the MSB
    tax                      ; ...and use the result as our loop counter
    iny                      ; Move on to next byte in data structure
    lda (LO_ADDR),y          ; Get Value from data structure
:   sta $2006                ; Write it $2006
    dex                      ; Decrement the iteration count (for this value)
    bne :-                   ; ...and repeat writing until 0
    pla                      ; Restore the original Count value
    bmi Done                 ; If MSB is set (negative flag set), we're done
    lda LO_ADDR              ; ...otherwise increment LO_ADDR (low byte) by 2
    clc
    adc #2
    sta LO_ADDR              ; ...if the ADC set the carry (indicating unsigned
    bcc :+                   ; ...overflow), then we know we wrapped ($FF->00)
    inc LO_ADDR+1            ; ...and need to increment the high byte
:   jmp L1                   ; Move on to the next entry
Done:
    rts                      ; ...and we're completely done
.endproc