path: root/src/turboload.s

;; Modified Topo soft loader
;;
;; Multi-colour bars in the border, aka "Spanish" style Spectrum variant loader.
;;
;; This loader uses standard Spectrum ROM timings. In fact it is essentially a CPC version of the
;; Spectrum ROM loader itself.
;;
;;
;; Loader form on cassette:
;; Pilot tone (min 256 pulses)
;; Sync pulse
;; 1 byte sync byte (on spectrum 0 for header and &ff for data)
;; n bytes of data (defined by DE register)
;; 1 byte parity checksum
;;
;; Original had the following bugs:
;; 1. bit 7 of R register had to be 1, otherwise it could change ram/rom configuration while loading
;; 2. flags were not returned, so you didn't know if the data was read incorrectly or not
;;
;; It also had extra code that was unnecessary:
;; 1. turning cassette motor on/off was complex, when in previous instructions they had turned it on anyway.
;; 2. code for changing border colour presumably on a read error. but didn't seem to be used.
;;
;;
;; Entry conditions:
;;
;; IX = start
;; DE = length (D must not be &ff)
;; A = sync byte expected
;;
;; Interrupts must be disabled
;;
;; Exit Conditions:
;; Alternative register set is corrupted. 
;;
;; carry clear - load ok
;; carry set, zero set - time up
;; carry set, zero reset - if esc pressed
;;
;; Use 2CDT to write Spectrum ROM blocks to a CDT. Note that at this time it will only write a sync byte of &ff.

_turboload:

inc     d					;; reset the zero flag (D cannot hold &ff)
ex      af,af'				;; A register holds sync byte. 
dec     d					;; restore D
exx
;; we need B' to be so we can write to gate-array i/o port for colour change when loading

ld      bc,#0x7f00+#0x10		;; Gate-Array + border
out 	(c),c				;; select pen index to change while loading (&10 is border)
ld 		c,#0x54					;; set to black
out 	(c),c
exx

ld      bc,#0xf40e			;; select AY register 14 (for reading keyboard)
out     (c),c

ld      bc,#0xf600+#0xc0+#0x10	;; "AY write register" and enable tape motor
out     (c),c

ld      c,#0x10				;; "AY inactive" and enable tape motor (register is latched into AY)
out     (c),c

ld      bc,#0xf792			;; set PPI port A to read (so we can read keyboard data)
out     (c),c				;; this will also write 0 to port C and port A on CPC.

ld      bc,#0xf600+#0x40+#0x10+#0x8	;; tape motor enable, "AY read register", select keyboard row 8
							;; (keys on this row: z, caps lock, a, tab, q, esc, 2, 1)
							;; we are only interested in ESC
out     (c),c

;; make an initial read of cassette input
ld      a,#0xf5			;; PPI port B
in      a,(#0)			;; read port (tape read etc)
and     #0x80				;; isolate tape read data

ld      c,a
cp      a				;; set the zero flag
l8107: 
ret nz 					;; returns if esc key is pressed (was RET NZ)
l8108: 
call    l817b
jr      nc,l8107

;; the wait is meant to be almost one second
ld      hl,#0x415
l8110: 
djnz    l8110
dec     hl
ld      a,h
or      l
jr      nz,l8110
;; continue if only two edges are found within this allowed time period
call    l8177
jr      nc,l8107

;; only accept leader signal
l811c: 
ld      b,#0x9c
call    l8177
jr      nc,l8107
ld      a,#0xb9
cp      b
jr      nc,l8108
inc     h
jr      nz,l811c

;; on/off parts of sync pulse
l812b: 
ld      b,#0xc9
call    l817b
jr      nc,l8107
ld      a,b
cp      #0xd1
jr      nc,l812b
l8137: 
call    l817b
ret     nc

ld      h,#0			;; parity matching byte (checksum)
ld      b,#0xb0			;; timing constant for flag byte and data
jr      l815d

l8145: 
ex      af,af'			;; fetch the flags
jr      nz,l814d		;; jump if we are handling first byte
;; L = data byte read from cassette
;; store to RAM
ld      0(ix),l
jr      l8157

l814d: 
rr      c				;; keep carry in safe place
						;; NOTE: Bit 7 is cassette read previous state
						;; We need to do a right shift so that cassette read moves into bit 6,
						;; our stored carry then goes into bit 7
xor     l				;; check sync byte is as we expect
ret     nz

ld      a,c				;; restore carry flag now
rla     				;; bit 7 goes into carry restoring it, bit 6 goes back to bit 7 to restore tape input value
ld      c,a
inc     de			;; increase counter to compensate for it's decrease
jr      l8159

l8157: 
inc     ix			;; increase destination
l8159: 
dec     de			;; decrease counter
ex      af,af'		;; save the flags

ld      b,#0xb2		;; timing constant
l815d: 
ld      l,#1		;; marker bit (defines number of bits to read, and finally becomes value read)
l815f: 
call    l8177
ret     nc

ld      a,#0xc3		;; compare the length against approx 2,400T states, resetting the carry flag for a '0' and setting it for a '1'
cp      b
rl      l			;; include the new bit in the register
ld      b,#0xb0		;; set timing constant for next bit
jr      nc,l815f

;; L = data byte read
;; combine with checksum
ld      a,h
xor     l
ld      h,a

;; DE = count
ld      a,d
or      e
jr      nz,l8145

exx
ld 		c,#0x54						;; make border black
out 	(c),c
exx

ld      bc,#0xf782				;; set PPI port A for output
out     (c),c
ld      bc,#0xf600				;; tape motor off, "AY inactive"
out     (c),c

;; H = checksum byte
;; H = 0 means checksum ok
ld      a,h
cp      #1
;; return with carry flag set if the result is good
;; carry clear otherwise
ret     


;;------------------------------------------------------------
l8177: 
call    l817b			;; in effect call ld-edge-1 twice returning in between if there is an error
ret     nc

;; wait 358T states before entering sampling loop
l817b: 
ld      a,#0x16			;; [2]
l817d: 
dec     a				;; [1]
jr      nz,l817d		;; ([3]+[1])*&15+[2]

and     a
l8181: 
inc     b				;; count each pass
ret     z				;; return carry reset and zero set if time up.

;; read keyboard
ld      a,#0xf4			;; PPI port A 
in      a,(#0)			;; read port and read keyboard through AY register 14
and     #0x04				;; isolate state of bit 2 (ESC key)
						;; bit will be 0 if key is pressed
xor     #0x04				;; has key been pressed? 
ret     nz				;; quit (carry reset, zero reset)

ld      a,#0xf5			;; PPI port B
in      a,(#0)			;; read port (tape read etc)
xor     c
and     #0x80				;; isolate tape read
jr      z,l8181
ld      a,c				;; effectively toggle bit 7 of C
cpl     				;; which is the tape read state we want to see next
ld      c,a

exx     				;; swap to alternative register set so that we can change colour
;; this sets colour and gives us the multi colour border
;ld 		a,r				;; get R register
;and 	#0x1f					;; ensure it is in range of colour value

nop
nop
nop
turbo_col:
and		#4

or 		#0x40					;; bit 7 = 0, bit 6 = 1
out 	(c),a				;; write colour value
nop						;; this is to ensure the number of cycles is the same
						;; compared to the replaced instructions
						;; (timings from toposoft loader)
nop
exx
scf
ret