implemented trap #0 handler in assembler

sources/xhdi_vec.S

@@ -3,13 +3,36 @@
 //
 		.extern	_xhdi_call
 		.globl	_xhdi_vec
+		.globl	_xhdi_sd_install
 
+//
+// this is where the XHDI cookie points to:
+//
+
+		.text
 _xhdi_vec:
-		lea     -12(sp),sp
+		lea     -12(sp),sp		// save all used registers according to XHDI spec
 		movem.l d1/a0-a1,(sp)
-		pea     16(sp)
-		jsr		_xhdi_call
-		addq.l  #4,sp
-		movem.l (sp),d1/a0-a1
+		pea     16(sp)			// forward address of parameters on stack
+		jsr		_xhdi_call		// to internal routine
+		addq.l  #4,sp			// correct stack
+		movem.l (sp),d1/a0-a1	// restore registers
 		lea     12(sp),sp
 		rts
+
+		.equ	_drvbits, 0x4c2
+
+		.data
+_old_vector:
+		.ds.l	1
+
+		.text
+
+//
+// trap #0 handler to bring the address of the disk routines into TOS
+//
+_xhdi_sd_install:
+		move.l	4(sp),d0		// address of the old XHDI vector
+		move.l	d0,_old_vector	// save it in case we need it
+		move.l	_xhdi_vec,d0	// return our BaS vector to TOS
+		rte