acia: simplify driver

The previous approach of maintaining R> and W> pointers was
conceptually simple, but made INT handler code actually quite
complex.

Now, we maintain indexes instead. It's much easier to perform
bounds checks and to compare for equality, something we have to
do quick in the INT handler.

blk/208

@@ -5,7 +5,7 @@ d => BC DE HL AF/SP
 c => CNZ CZ CNC CC CPO CPE CP CM
 
 LD [rr, ri, di, (i)HL, HL(i), d(i), (i)d, rIXY, IXYr,
-    (DE)A, A(DE)]
+    (DE)A, A(DE), (i)A, A(i)]
 ADD [r, i, HLd, IXd, IXIX, IYd, IYIY]
 ADC [r, HLd]
 CP  [r, i, (IXY+)]

blk/240

@@ -7,5 +7,5 @@
 ;
 0xcd OP3n CALL,
 0xc3 OP3n JP,
-0x22 OP3n LD(n)HL,
-0x2a OP3n LDHL(n),
+0x22 OP3n LD(n)HL,             0x2a OP3n LDHL(n),
+0x32 OP3n LD(i)A,              0x3a OP3n LDA(i),

blk/582

@@ -1,15 +1,14 @@
 0x80 CONSTANT ACIA_CTL ( IO port for ACIA's control register )
 0x81 CONSTANT ACIA_IO ( IO port for ACIA's data registers )
+0x20 CONSTANT ACIA_BUFSZ ( SZ-1 must be a mask )
 ( Address in memory that can be used variables shared
-  with ACIA's native words. 8 bytes used. )
+  with ACIA's native words. 4 bytes used. )
 CREATE ACIA_MEM SYSVARS 0x70 + ,
 ( Points to ACIA buf )
-: ACIA( ACIA_MEM @ 4 + ;
-( Points to ACIA buf end )
-: ACIA) ACIA_MEM @ 6 + ;
-( Read buf pointer. Pre-inc )
+: ACIA( ACIA_MEM @ 2+ ;
+( Read buf idx Pre-inc )
 : ACIAR> ACIA_MEM @ ;
-( Write buf pointer. Post-inc )
-: ACIAW> ACIA_MEM @ 2+ ;
+( Write buf idx Post-inc )
+: ACIAW> ACIA_MEM @ 1+ ;
 ( This means that if W> == R>, buffer is full.
   If R>+1 == W>, buffer is empty. )

blk/583

@@ -1,13 +1,16 @@
-(entry) ~ACIA ( Set RST 38 jump ) PC ORG @ 0x39 + !
-    AF PUSH, HL PUSH, DE PUSH,
-	( Read our character from ACIA into our BUFIDX )
-    ACIA_CTL INAi,
-    0x01 ANDi,   ( is ACIA rcv buf full? )
-    IFNZ,
-        ( correct interrupt cause )
-        ACIAW> LDHL(n),
-        ( is it == to ACIAR>? )
-        ( +0 == ACIAR> )
-        DE ACIAR> LDd(n),
-        ( carry cleared from ANDi above )
-        DE SBCHLd,                                    ( cont. )
+( ACIA INT handler, read into ACIAW> )
+( Set RST 38 jump ) PC ORG @ 0x39 + !
+    AF PUSH,
+    ACIA_CTL INAi, 0x01 ANDi, ( is ACIA rcv buf full? )
+    IFZ, ( no, abort ) AF POP, EI, RETI, THEN,
+    HL PUSH,
+    HL ACIAW> LDdn, A (HL) LDrr,
+    HL DECd, (HL) CPr, ( W> == R> ? )
+    IFNZ, ( buffer not full )
+        ( get wr ptr ) HL ACIA( LDd(n),
+        L ADDr, IFC, H INCr, THEN, L A LDrr,
+        ( fetch/write ) ACIA_IO INAi, (HL) A LDrr,
+        ( advance W> ) ACIAW> LDA(i), A INCr,
+        ACIA_BUFSZ 1- ANDi, ACIAW> LD(i)A,
+    THEN,
+    HL POP, AF POP, EI, RETI,

blk/584

@@ -1,14 +0,0 @@
-        IFNZ, ( buffer full? )
-            ( no, continue )
-            DE ADDHLd, ( restore ACIAW> )
-            ( buffer not full, let's write )
-            ACIA_IO INAi,
-            (HL) A LDrr,
-            ( advance W> )
-            HL INCd,
-            ACIAW> LD(n)HL,
-            DE ACIA) LDd(n),
-            DE SUBHLd,
-
-
-                                                      ( cont. )

blk/585

@@ -1,9 +0,0 @@
-            IFZ, ( end of buffer reached? )
-                ( yes )
-                ACIA( LDHL(n),
-                ACIAW> LD(n)HL,
-            THEN,
-        THEN,
-    THEN,
-    DE POP, HL POP, AF POP,
-    EI, RETI,

blk/587

@@ -1,12 +1,10 @@
 : (key)
     ( inc then fetch )
-    [ ACIAR> LITN ] @ 1+ DUP [ ACIA) LITN ] @ = IF
-        DROP [ ACIA( LITN ] @
-    THEN
+    [ ACIAR> LITN ] C@ 1+ [ ACIA_BUFSZ 1- LITN ] AND
     ( As long as R> == W>-1, it means that buffer is empty )
-    BEGIN DUP [ ACIAW> LITN ] @ = NOT UNTIL
-    [ ACIAR> LITN ] !
-    [ ACIAR> LITN ] @ C@
+    BEGIN DUP [ ACIAW> LITN ] C@ = NOT UNTIL
+    DUP [ ACIA( LITN ] @ + C@ ( ridx c )
+    SWAP [ ACIAR> LITN ] C! ( c )
 ;
 : (emit)
     ( As long at CTL bit 1 is low, we are transmitting. wait )

blk/588

@@ -1,8 +1,7 @@
 : ACIA$
-    H@ DUP DUP [ ACIA( LITN ] ! [ ACIAR> LITN ] !
-    1+ [ ACIAW> LITN ] ! ( write index starts one pos later )
-    0x20 ( buffer size ) ALLOT
-    H@ [ ACIA) LITN ] !
+    H@ [ ACIA( LITN ] ! 0 [ ACIAR> LITN ] C!
+    1 [ ACIAW> LITN ] C! ( write index starts one pos later )
+    [ ACIA_BUFSZ LITN ] ALLOT
 ( setup ACIA
   CR7 (1) - Receive Interrupt enabled
   CR6:5 (00) - RTS low, transmit interrupt disabled.