z80a: add BREAK, instruction

This allows us to remove a lot of labels usage in boot code. This
commit has no effect on forth.bin.

blk/204

@@ -8,3 +8,9 @@ byte that is taken care of. We still need to adjust by another
 byte before writing the offset.
 
 
+
+
+
+
+
+                                                        (cont.)

blk/205

@@ -0,0 +1,16 @@
+Structured flow
+
+z80a also has words that behave similarly to IF..THEN and
+BEGIN..UNTIL.
+
+On the IF side, we have IFZ, IFNZ, IFC, IFNC, and THEN,. When
+the opposite condition is met, a relative jump is made to
+THEN,'s PC. For example, if you have IFZ, a jump is made when
+Z is unset.
+
+On the BEGIN,..AGAIN, side, it's a bit different. You start
+with your BEGIN, instruction, and then later you issue a
+JRxx, instr followed by AGAIN,. Exactly like you would do
+with a label.
+
+                                                        (cont.)

blk/206

@@ -0,0 +1,3 @@
+On top of that, you have the very nice BREAK, instruction,
+which must also be preceeded by a JRxx, and will jump to the
+PC following the next AGAIN,

blk/246

@@ -1,13 +1,16 @@
 ( Place BEGIN, where you want to jump back and AGAIN after
   a relative jump operator. Just like BSET and BWR. )
 : BEGIN, PC ;
-: AGAIN, PC - 1- A, ;
-
 : BSET PC SWAP ! ;
-: BWR @ AGAIN, ;
 ( same as BSET, but we need to write a placeholder )
 : FJR, PC 0 A, ;
 : IFZ, JRNZ, FJR, ;
 : IFNZ, JRZ, FJR, ;
 : IFC, JRNC, FJR, ;
 : IFNC, JRC, FJR, ;
+: THEN,
+    DUP PC          ( l l pc )
+    -^ 1-           ( l off )
+    ( warning: l is a PC offset, not a mem addr! )
+    SWAP ORG @ + BIN( @ - ( off addr )
+    C! ;

blk/247

@@ -1,9 +1,8 @@
-: THEN,
-    DUP PC          ( l l pc )
-    -^ 1-           ( l off )
-    ( warning: l is a PC offset, not a mem addr! )
-    SWAP ORG @ + BIN( @ - ( off addr )
-    C!
-;
 : FWR BSET 0 A, ;
 : FSET @ THEN, ;
+: BREAK, FJR, 0x8000 OR ;
+: BREAK?, DUP 0x8000 AND IF
+        0x7fff AND 1 ALLOT THEN, -1 ALLOT
+    THEN ;
+: AGAIN, BREAK?, PC - 1- A, ;
+: BWR @ AGAIN, ;

blk/291

@@ -13,4 +13,4 @@ PC ORG @ 4 + ! ( find )
     NEG,
     A DECr,
 	( special case. zero len? we never find anything. )
-    JRZ, L1 FWR ( fail-B296 )                         ( cont. )
+    IFNZ, ( fail-B296 )                               ( cont. )

blk/293

@@ -9,8 +9,7 @@
                 HL DECss,
                 LDA(DE),
                 (HL) CPr,
-                JRNZ, L2 FWR ( loopend )
+                JRNZ, BREAK,
             DJNZ, AGAIN,
-            L2 FSET ( loopend )
         THEN,
                                                       ( cont. )

blk/296

@@ -1,4 +1,4 @@
-    L1 FSET ( fail )
+    THEN, ( zero length check, B291 )
     A XORr,
     A INCr,
     L2 FSET ( end )

blk/328

@@ -2,15 +2,13 @@ CODE S=
     DE  POPqq,
     HL  POPqq,
     chkPS,
-    BEGIN, ( loop )
+    BEGIN,
         LDA(DE),
         (HL) CPr,
-        JRNZ, L1 FWR ( not equal? break early to "end".
+        JRNZ, BREAK, ( not equal? break early. NZ is set. )
-                       NZ is set. )
         A ORr,   ( if our char is null, stop )
         HL INCss,
         DE INCss,
-    JRNZ, AGAIN, ( loop )
+    JRNZ, AGAIN,
-L1 FSET ( end )
     PUSHZ,
 ;CODE