open-roms/src/kernal/tape/load_tape_normal.s

;; #LAYOUT# STD *        #TAKE
;; #LAYOUT# M65 KERNAL_1 #TAKE
;; #LAYOUT# *   *        #IGNORE

;
; Tape (normal) part of the LOAD routine
;


; We have the following possible CPU frequencies:
; - C64    PAL:  CPU frequency 0.985248 MHz
; - C64    NTSC: CPU frequency 1.022727 MHz
; The clock difference is about 4% - it is unlikely to cause problems.
;
; We have to distinguish the following pulse lengths (http://sidpreservation.6581.org/tape-format/):
; - S (short)    tap value $30 (PAL: 352 us)
; - M (medium)   tap value $42 (PAL: 512 us)
; - L (long)     tap value $56 (PAL: 672 us)
;
; The TAP format (http://unusedino.de/ec64/technical/formats/tap.html) uses a resolution of 8 PAL ticks;
; this implementation sets the timer for twice the precision (timer B is triggered every 4 CPU cycles),
; probably an overkill, but in realityy it does not cost us anything.
;
; So - we have the following thresholds:
; - S vs M       $72 (in our units of 4 ticks)
; - M vs L       $98 (in our units of 4 ticks)
;
; We need to recognize the following pulse sequences:
;
; (S,M) = 0 bit
; (M,S) = 1 bit
; (L,M) = new-data marker
; (L,S) = end-of-data marker


!ifdef CONFIG_TAPE_NORMAL {


!ifndef CONFIG_TAPE_AUTODETECT {

load_tape_normal:

	jsr tape_ditch_verify              ; only LOAD is supported, no VERIFY

	; Start playing
	jsr tape_common_prepare_cia
	jsr tape_ask_play
!ifdef CONFIG_TAPE_HEAD_ALIGN {
	jsr tape_prepare_reading
}

	; FALLTROUGH

load_tape_normal_header:

} else {

load_tape_normal_takeover:             ; entry point for turbo->normal takeover
}

	jsr load_MEMUSS_to_STAL

	; Try to load header into tape buffer (we will restore MEMUSS later)

	jsr tape_normal_get_pilot_header
!ifdef CONFIG_TAPE_AUTODETECT {
	bcs load_tape_normal_switch_turbo  ; failed, try turbo
}

	jsr load_TAPE1_to_MEMUSS
	jsr tape_normal_get_data_1
!ifndef CONFIG_TAPE_AUTODETECT {
	bcs load_tape_normal_header        ; unable to read block, try again
} else {
	bcc load_tape_normal_switch_turbo_skip
load_tape_normal_switch_turbo:
	jsr lvs_STAL_to_MEMUSS
	jmp load_tape_turbo_takeover       ; unable to read block, try turbo instead
load_tape_normal_switch_turbo_skip:
}

	jsr load_TAPE1_to_MEMUSS
	jsr tape_normal_get_data_2

!ifndef CONFIG_TAPE_AUTODETECT {
	bcs load_tape_normal_header        ; block load error, try again
} else {
	bcs load_tape_normal_switch_turbo  ; block load error, try turbo instead
}

	; Check header type

	ldy #$00
	lda (TAPE1),y

	cmp #$05
	+beq tape_load_error             ; end of tape mark, nothing to load

	cmp #$01
	beq @1
	cmp #$03
!ifndef CONFIG_TAPE_AUTODETECT {
	bne load_tape_normal_header        ; header types 1 and 3 are loadable, see Mapping the C64, page 77
} else {
	bne load_tape_normal_takeover
}

	lda #$01
	sta SA                             ; this file is non-relocatable; override secondary address with 1
@1:
	; Restore MEMUSS, handle header

	jsr lvs_STAL_to_MEMUSS
	jsr tape_handle_header
!ifndef CONFIG_TAPE_AUTODETECT {
	bcs load_tape_normal_header        ; if name does not match, look for other header
} else {
	bcs load_tape_normal_takeover
}

	; FALLTROUGH

load_tape_normal_payload:

	; Retrieve data

	jsr tape_normal_get_pilot_data
	ldy #$00                           ; no data size limit
	jsr tape_normal_get_data_1
	+bcs tape_load_error

	jsr lvs_check_EAL
	+bne tape_load_error             ; amount of data read does not match header info

	jsr lvs_STAL_to_MEMUSS
	jsr tape_normal_get_data_2

	+bcs tape_load_error

	jmp tape_load_success



load_TAPE1_to_MEMUSS:                   ; set MEMUSS (start address) as tape buffer 

	lda TAPE1+1
	sta MEMUSS+1
	lda TAPE1+0
	sta MEMUSS+0

	rts

load_MEMUSS_to_STAL:                   ; preserve MEMUSS

	lda MEMUSS+1
	sta STAL+1
	lda MEMUSS+0
	sta STAL+0

	rts
}