Simplify bootable ramdisk detection. Jump to AppleSoft if no boot device.

I discovered that there are more than one ProDOS boot block in the wild, so I
simplified the check for recoverable RAM disk to check card for $01 and then
anything besides $00 and $FF since most uninitialized RAM tends to have one of
those two values.  This is slightly more rigid than the boot code checks for.
I added a jump to AppleSoft if no bootable device is found.  I changed the
message a little bit, too.
I also added notes about the feasibility of using the RamFactor firmware.

notes/ramfactor.txt

@@ -0,0 +1,70 @@
+A potential possibility is to replace Apple Slinky firmware with the RamFactor firmware.
+
+The ROM of the RamFactor is 8K in size, but that is not how much we'd need in the IIc.
+
+The layout of the ROM is two banks of 4K, selected by the low bit of $C08F+slot*$10.
+
+Each bank is laid out as follows, with the base at $C000
+$C000-$C0FF: Copyright message and fill, not visible in-system.
+$C100-$C7FF: Slot firmware, $100 bytes for each slot, same code in both banks
+             This represents the same code assembled for each slot.
+             Only the slot the card is in is visible.
+$C800-$CFFF: 2K of shared firmware space firmware, different for each bank.
+
+So as we see, without any modifications we need approximately 4K+256 bytes of space to
+accomodate the RamFactor code.  It is likely we will need to do some bank switching which
+tends to add code, but also make some optimizations or drop features, which will remove
+code.
+
+There is definitely enough space in the //c firmware for it.  Especially if we can drop
+the original slinky code, which uses $C400-$C4FF, $C752-$C762 (bank switch, both banks),
+and $D800-$DB58 (eff. $DBFF).  $C763-$C7FF is already used by ROM 4X.
+
+Features that might be droppable:  More than 1MB memory support.
+
+Steps needed:
+
+Produce commented RF source that assembles to a binary match for the latest firmware (1.4).
+
+Port to Apple //c:
+  - The main issue is that we don't have the ability to run RF code at $C800.
+  - Bank switch code will need to be in the C400 space, but in the alt bank that
+    has the //c diagnostics, which will either need to be modified or we will have to
+    use less efficient bank switching (RTS trick).  We can optimize the RTS trick in this
+    scenario by having an entry point table all on the same page.  If we waste a few bytes
+    in the alt firmware bank we can do something like:
+entry:	; expect x = function code
+	lda table_hibyte
+	pha
+	txa
+	asl
+	asl ; mult by 4
+	pha ; "return addr" now a multiple of 4 in jump table
+	jmp swrts ; switch bank and go
+  - If the RF code calls any monitor routines we may need to supply them outright
+    or provide for a bank-switched call.
+  - We are likely going to need a "cross-bank JSR".
+    - Caller:  Push callee address-1 on stack and JSR to xb_jsr
+    - xb_jsr:  change banks, stack has: SP->caller callee
+               rework stack so that we have the following return entries
+               SP->callee swrts caller
+               sta $       ; into screen holes
+               stx $
+               tsx         ; SP->caller callee, x->caller callee
+               lda $103,x  ; callee high 
+               pha         ; SP->callee[h] caller callee
+               lda $102,x  ; callee low
+               pha         ; SP->callee caller callee
+               lda $101,x  ; caller high
+               sta $103,x  ; SP->callee caller callee[l]+caller[h]
+               lda $100,x  ; caller low
+               sta $102,x  ; SP->callee caller caller
+               lda #swrts_hi-1
+               sta $101,x
+               lda #swrts_lo-1
+               sta $100,x  ; SP->callee swrts caller
+               lda $       ; from screen holes
+               ldx $
+               rts         ; pops callee, callee rts pops swrts, swrts pops caller
+               ; above code is ~44 bytes
+

rom4x/B0_C552_patch_bootfail.s

@@ -0,0 +1,18 @@
+#include "iic.defs"
+
+.text
+* = $c552
+	jsr setnorm
+	jsr init
+	bra cbtfail
+	.dsb coma-*,$ea
+	bra coma		; Make sure coma routine exists
+	.db 0			; rom4x present
+cbtfail	jsr setvid
+	jsr setkbd
+	lda #>(nbtfail-1)
+	pha
+	lda #<(nbtfail-1)
+	pha
+	jmp swrts2
+	

rom4x/B1_E000_rom4x.s

@@ -161,15 +161,13 @@ rdrecov	jsr rdinit		; init ramcard
 	stz addrl,x		; set slinky address 0
 	stz addrm,x
 	stz addrh,x
-	lda data,x		; start compare to ProDOS boot block
+	lda data,x		; start check for bootable ramdisk
 	cmp #$01
-	bne recovdn		; not ProDOS
-	lda data,x
-	cmp #$38
-	bne recovdn		; not ProDOS
-	lda data,x
-	cmp #$b0
-	bne recovdn		; not ProDOS
+	bne recovdn		; not bootable
+	lda data,x		; next byte should be nonzero and not $ff
+	beq recovdn		; not bootable
+	cmp #$ff
+	beq recovdn		; not bootable
 	lda #pwrbyte
 	sta pwrup,y		; set power byte
 	lda "R"			; tell user

rom4x/B1_E300_new_bootfail.s

@@ -0,0 +1,19 @@
+#include "iic.defs"
+.text
+* = nbtfail
+	ldx #msglen
+lp1	lda bootmsg,x
+	ora #$80
+	sta $7d0+19-msglen/2,x
+	dex
+	bpl lp1
+	lda #23		; last line
+	sta cv
+	lda #>(basic-1)
+	pha
+	lda #<(basic-1)
+	pha
+	jmp swrts2
+bootmsg	.db "No bootable device."
+msglen = * - bootmsg - 1
+

rom4x/iic.defs

@@ -30,6 +30,7 @@ sl_devno = $778
 ; locations
 softev = $3f2
 pwerdup = $3f4
+cv = $25
 
 ; values
 pwrbyte = $a5
@@ -46,8 +47,15 @@ rst4xrtn = $facb	; where to return from reset4x
 bt4xrtn = $fb19		; where to return from boot4x
 reset4x = $e000		; reset routine
 boot4x = $e180		; boot routine
+nbtfail = $e300		; new boot fail
 banner = $fb60		; 'Apple //c'
 testsize = $d99f	; test ramdisk size (rom 3 = $d995)
 monitor = $ff59		; monitor
 swrts2 = $c784		; switch firmware and rts
+basic = $e000		; BASIC cold start entry point
+coma = $c55d		; location of coma routine
+setnorm = $fe84		; normal video
+init = $fb2f		; init screen
+setvid = $fe93		; init output
+setkbd = $fe89		; init input