73 lines
1.3 KiB
ArmAsm
73 lines
1.3 KiB
ArmAsm
;; #LAYOUT# STD * #TAKE
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;; #LAYOUT# CRT KERNAL_0 #TAKE
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;; #LAYOUT# M65 KERNAL_1 #TAKE
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;; #LAYOUT# * * #IGNORE
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;
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; Tape (normal) helper routine - byte reading
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;
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; Returns byte in .A (also sotred in INBIT), Carry flag set = error
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;
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!ifdef CONFIG_TAPE_NORMAL {
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tape_normal_get_byte:
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; Init parity bit and canary bit
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lda #$01
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sta PRTY
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lda #%01111111 ; canary bit is 0
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sta INBIT
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; Now fetch individual bits
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tape_normal_get_byte_loop:
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jsr tape_normal_get_bit
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bcs tape_normal_get_byte_error
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beq @1
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; Handle parity bit
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lda #$01
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eor PRTY
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sta PRTY
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sec
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@1: ; moved bit state from Zero to Carry flag
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ror INBIT ; put the bit in
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bcs tape_normal_get_byte_loop ; loop if no canary bit reached
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; Byte retrieved (on stack), now we need to validate the parity
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jsr tape_normal_get_bit
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bcs tape_normal_get_byte_error
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beq @2
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lda #$01
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@2:
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eor PRTY
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bne tape_normal_get_byte_error
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; Checksum validated succesfully
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ldx #$0B
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; Carry already clear
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; FALLTROUGH
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tape_normal_get_byte_done:
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stx VIC_EXTCOL
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lda INBIT
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rts
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tape_normal_get_byte_error:
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ldx #$09
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sec ; make sure Carry is set to indicate error
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bcs tape_normal_get_byte_done
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}
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