add 64K check

src/4cade.a

@@ -5,10 +5,24 @@
          !source "src/constants.a"
          !source "src/macros.a"
 
+         jsr   Has64K                ; check for 64K (required)
+         bcc   @found64K
+         jsr   $FB2F
+         jsr   $FC58
+         ldy   #@no64Klen
+-        lda   @s_no64K,y
+         sta   $6B6,y
+         dey
+         bpl   -
+@hang    bmi   @hang
+@s_no64K !raw  "REQUIRES 64K"
+@no64Klen=*-@s_no64K
+
+@found64K
          jsr   DisableAccelerator    ; set to 1 MHz
-         jsr   Has128K               ; check for 128K (64K is OK, we just filter out some games)
+         jsr   Has128K               ; check for 128K (absence is OK, we just filter out some games)
          ror   MachineStatus
-         jsr   HasJoystick           ; check for joystick (keyboard-only is OK, we just filter out some games)
+         jsr   HasJoystick           ; check for joystick (absence is OK, we just filter out some games)
          ror   MachineStatus
 
          lda   LCBANK1WRITE          ; read ROM / write LC bank 1

src/memcheck.a

@@ -1,3 +1,30 @@
+;------------------------------------------------------------------------------
+; Has64K
+; Checks whether computer has functioning language card (64K)
+;
+; in:    none
+; out:   C set if 64K detected
+;        C clear if 64K not detected
+;        all other flags and registers clobbered
+;        ROM in memory (not LC RAM bank)
+;------------------------------------------------------------------------------
+Has64K
+         lda   LCBANK1               ; read/write LC RAM bank 1
+         lda   LCBANK1
+         lda   #$AA                  ; test #1 for $D0 page
+         sta   $D000
+         eor   $D000
+         bne   @no
+         lsr   $D000                 ; test #2 for $D0 page
+         lda   #$55
+         eor   $D000
+         bne   @no
+         clc
+         +HIDE_NEXT_BYTE
+@no      sec
+         sta   ROMONLY
+         rts
+
 ;------------------------------------------------------------------------------
 ; Has128K
 ; Checks whether computer has functioning auxiliary memory (128K)
@@ -13,7 +40,6 @@
 ; (c) 1989 Broderbund Software
 ; https://github.com/jmechner/Prince-of-Persia-Apple-II/blob/master/01%20POP%20Source/Source/BOOT.S#L119
 ;------------------------------------------------------------------------------
-!zone {
 Has128K
          sta   ROMONLY               ; need ROM for machine ID byte
          lda   MACHINEID
@@ -49,4 +75,3 @@ Has128K
          sta   READMAINMEM
          rts
 @checklen=*-@checker
-}