z80: Use BC as IP register instead of IY

It's a bit more inconvenient in terms of register protection (BC
is much more generally useful than IY), but it makes tight spots
such as next and execute much faster, so I think it's worth it.

blk/280

@@ -4,8 +4,8 @@ This assembles the boot binary. It requires the Z80 assembler
 (B200) and cross compilation setup (B260). It requires some
 constants to be set. See B420 for details.
 
-RESERVED REGISTERS: At all times, IX points to RSP TOS and IY
-is IP. SP points to PSP TOS, but you can still use the stack\
+RESERVED REGISTERS: At all times, IX points to RSP TOS and BC
+is IP. SP points to PSP TOS, but you can still use the stack
 in native code. you just have to make sure you've restored it
 before "next".
 

blk/286

@@ -10,7 +10,6 @@ lblnext BSET PC ORG @ 0x1b + ! ( next )
     DE RS_ADDR LDdn,
     DE SUBHLd,
     JRC, lblofl BWR ( IX < RS_ADDR? abortUnderflow-B285 )
-    E 0 IY+ LDrIXY,
-    D 1 IY+ LDrIXY,
-    IY INCd, IY INCd,
+    LDA(BC), E A LDrr, BC INCd,
+    LDA(BC), D A LDrr, BC INCd,
     ( continue to execute )

blk/289

@@ -2,13 +2,12 @@
 ( 1. Push current IP to RS
   2. Set new IP to the second atom of the list
   3. Execute the first atom of the list. )
-    IY PUSH, HL POP, ( <-- IP )
     IX INCd, IX INCd,
-    0 IX+ L LDIXYr,
-    1 IX+ H LDIXYr,
+    0 IX+ C LDIXYr,
+    1 IX+ B LDIXYr,
     EXDEHL,          ( HL points to PFA )
 ( While we inc, dereference into DE for execute call later. )
     LDDE(HL),
     HL INCd,
-    HL PUSH, IY POP, ( --> IP )
+    B H LDrr, C L LDrr, ( --> IP )
     JR, lblexec BWR      ( execute-B287 )

blk/297

@@ -1,9 +1,9 @@
 CODE (br)
 L1 BSET ( used in ?br and loop )
 PC ORG @ 0x3d + ! ( stable ABI JP )
-    E 0 IY+ LDrIXY, D 0 LDri,
-    7 E BIT, IFNZ, D DECr, THEN,
-    DE ADDIYd,
+    LDA(BC), H 0 LDri, L A LDrr,
+    RLA, IFC, H DECr, THEN,
+    BC ADDHLd, B H LDrr, C L LDrr,
 ;CODE
 CODE (?br)
 PC ORG @ 0x41 + ! ( stable ABI JP )
@@ -11,5 +11,5 @@ PC ORG @ 0x41 + ! ( stable ABI JP )
     HLZ,
     JRZ, L1 BWR ( br + 1. False, branch )
     ( True, skip next byte and don't branch )
-    IY INCd,
+    BC INCd,
 ;CODE

blk/298

@@ -6,7 +6,7 @@ PC ORG @ 0x45 + ! ( stable ABI JP )
     A 1 IX+ LDrIXY, 1 IX- CP(IXY+), JRNZ, L1 BWR ( branch )
     ( don't branch )
     IX DECd, IX DECd, IX DECd, IX DECd,
-    IY INCd,
+    BC INCd,
 ;CODE
 
 

blk/305

@@ -5,8 +5,7 @@ CODE EXECUTE
 
 CODE EXIT
 PC ORG @ 0x0d + ! ( stable ABI JP )
-    L 0 IX+ LDrIXY,
-    H 1 IX+ LDrIXY,
+    C 0 IX+ LDrIXY,
+    B 1 IX+ LDrIXY,
     IX DECd, IX DECd,
-    HL PUSH, IY POP, ( --> IP )
     JPNEXT,

blk/306

@@ -3,8 +3,7 @@ PC ORG @ 0x25 + ! ( stable ABI JP )
   ( Literal value to push to stack is next to (n) reference
     in the atom list. That is where IP is currently pointing.
     Read, push, then advance IP. )
-    E 0 IY+ LDrIXY,
-    D 1 IY+ LDrIXY,
-    IY INCd, IY INCd,
-    DE PUSH,
+    LDA(BC), L A LDrr, BC INCd,
+    LDA(BC), H A LDrr, BC INCd,
+    HL PUSH,
 ;CODE

blk/307

@@ -3,9 +3,9 @@ PC ORG @ 0x2d + ! ( stable ABI JP )
 ( Like (n) but instead of being followed by a 2 bytes
   number, it's followed by a string. When called, puts the
   string's address on PS )
-    IY PUSH, HL POP, ( <-- IP )
-    E (HL) LDrr, D 0 LDri,
-    DE INCd,
-    DE ADDIYd,
-    HL PUSH,
+    BC PUSH,
+    LDA(BC), C ADDr,
+    IFC, B INCr, THEN,
+    C A LDrr,
+    BC INCd,
 ;CODE

blk/308

@@ -1,6 +1,6 @@
 CODE ROT ( a b c -- b c a )
-    HL POP, ( C ) DE POP, ( B ) BC POP, ( A ) chkPS,
-    DE PUSH, ( B ) HL PUSH, ( C ) BC PUSH, ( A )
+    HL POP, ( C ) DE POP, ( B ) IY POP, ( A ) chkPS,
+    DE PUSH, ( B ) HL PUSH, ( C ) IY PUSH, ( A )
 ;CODE
 CODE DUP ( a -- a a )
     HL POP, chkPS,

blk/328

@@ -1,4 +1,4 @@
-CODE S=
+CODE S= EXX, ( protect BC )
     DE POP, HL POP, chkPS,
     LDA(DE),
     (HL) CPr,
@@ -12,4 +12,4 @@ CODE S=
         DJNZ, AGAIN,
     THEN,
     PUSHZ,
-;CODE
+EXX, ( unprotect BC ) ;CODE

blk/603

@@ -1,5 +1,5 @@
 ( Computes n into crc c with polynomial 0x1021 )
-CODE _crc16  ( c n -- c )
+CODE _crc16  ( c n -- c ) EXX, ( protect BC )
     HL POP, ( n ) DE POP, ( c )
     A L LDrr, D XORr, D A LDrr,
     B 8 LDri,
@@ -11,4 +11,4 @@ CODE _crc16  ( c n -- c )
         THEN,
     DJNZ, AGAIN,
     DE PUSH,
-;CODE
+EXX, ( unprotect BC ) ;CODE