Initial import for public releaseHEADmain

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+;; 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
+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
+