pcat: implement AT-XY

VE works! well, at this point, we can say that Collapse OS runs on
PC/AT. A little cleanup and docs writing and we're done!

recipes/pcat/xcomp.fs

@@ -50,6 +50,13 @@ CODE 13H ( ax bx cx dx -- ax bx cx dx )
     8 RSHIFT 1+ FDHEADS C!
     0x3f AND FDSPT C!
 ;
+: COLS 80 ; : LINES 25 ;
+CODE AT-XY ( x y )
+    ( DH=row DL=col BH=page )
+    AX POPx, BX POPx, DX PUSHx, ( protect )
+    DH AL MOVrr, DL BL MOVrr, BX BX XORxx, AH 2 MOVri,
+    0x10 INT, DX POPx, ( unprotect )
+;CODE
 380 LOAD  ( xcomp core high )
 (entry) _
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