Path: utzoo!utgpu!watserv1!watmath!att!pacbell!pacbell.com!ames!elroy.jpl.nasa.gov!usc!cs.utexas.edu!uunet!microsoft!alonzo
From: alonzo@microsoft.UUCP (Alonzo GARIEPY)
Newsgroups: comp.sys.handhelds
Subject: Machine code on the 28S (and 48SX)
Message-ID: <55022@microsoft.UUCP>
Date: 3 Jun 90 22:43:42 GMT
Reply-To: alonzo@microsoft.UUCP (Alonzo GARIEPY)
Organization: Microsoft Corp., Redmond WA
Lines: 126

In email, <Rick Grevelle> writes
| Hello Alonzo:
| 
| There are still many details about which I am confused. Can you tell me
| how to determine where to peek to view the code which is called by the 
| routine below? Even after studying your processor notes I'm unable to
| understand the instruction: 8Exxxx  CALL.4  PC+6+xxxx.
| 
| #44A4h:	#9A440
| 	#8E87A0         ????
| 	#147            move.a  @d1,c
| 	#137            swap.a   c,d1
| 	#141            move.a  a,@d1
| 	#135            move.a   c,d1
| 	#142            move.a  @d0,a
| 	#164            add.a    5,d0
| 	#808C           jump    @a
| 
| What did you mean when you said relative CALLS contain an offset from
| the next instruction?
| 
| And also, what did you mean when you said that these are expressed relative
| to the address of the call instruction itself (PC).
| 
| Would you explain what it means when you stated that offsets are
| in 2's complement form?
| 
| Lastly, how does the program counter enter in to: PC+6+xxxx? 
| 
| Thanks for any help you can give!
| 
| Rick

First, I have translated the listing to canonical form.  The answer to 
your primary question is evident from the listing: the called routine is
at #04f27.

	#044A4: #9A440		
	#044a9: #8E87A0 	call.4	#04f27 
	#044af: #00147  	move.a	@d1,c
	#044b2: #00137  	swap.a	c,d1
	#044b5: #00141  	move.a  a,@d1
	#044b8: #00135  	move.a	c,d1
	#044bb: #00142  	move.a	@d0,a
	#044be: #00164  	add.a	#5,d0
	#044c1: #00808C 	jump	@a

Now your other questions:

| What did you mean when you said relative CALLS contain an offset from
| the next instruction?

The jump instruction is described in the notes as
	6xxx	JUMP.3	PC+1+xxx

Take an example such as
	#00086:	#6634		jump.3	#004bd

In this case the PC is #00086, xxx is #436 (remember to reverse it) so the
destination is #86+1+#436 = #004bd.  The offset is relative to the second
nibble of the current instruction (i.e., PC + 1).

The call.4 instruction is described in the notes as
	8Exxxx	CALL.4	PC+6+xxxx

Take an example such as
	#03acd:	#8eea51		call.4 #05081

In this case the PC is #03acd, xxxx is #15ae (remember to reverse it) so the
destination is #03acd+6+#15ae = #05081.  The offset is relative to the first
nibble of the next instruction (for call.4 that means PC + 6).  In the Saturn 
CPU, all call offsets are relative to the first nibble of the next instruction.

| And also, what did you mean when you said that these are expressed relative
| to the address of the call instruction itself (PC).

For uniformity, I have written all address equations in terms of the current
PC.  Because the Saturn uses different base addresses to offset for jumps and
calls, I have specify correction factors such as +1, +2, and +6.  You can
see how these work from the above examples.

| Would you explain what it means when you stated that offsets are
| in 2's complement form?

That means that if the high bit is a 1, the offset is actually negative.
But there are at least three ways to write a negative number in binary.
The Saturn uses 2's complement form because the final address can be
calculated simply by sign extending the offset and adding it.  Sign extension
means taking the highest bit (1 or 0) and copying it all the way to the
left.

Take an example such as
	#00405: #6edf	jump.3	#003e4

Note that xxx is #fde.  In binary, that is 1111 1101 1110.  We can calculate
the final address (PC+1+xxx) by sign extending xxx and adding it to PC+1

	PC  0000 0000 0100 0000 0101 +	
	1   0000 0000 0000 0000 0001 +
	xxx 1111 1111 1111 1101 1110 =
	    ========================
   PC+1+xxx 0000 0000 0011 1110 0100  (extra carry is discarded)	

Note that both postive and negative numbers are said to be in 2's complement
form in this system and both are sign extended before the addition, but the
sign bit for postive numbers is 0 and it has no real effect.  For simpler,
manual subtraction, a 2's complement number can be negated by sign extending,
inverting all its bits and adding 1.

(1)	#fde
(2)	#fffde		(sign extend)
(3)	#00021		(invert)
(4)	#00022		(add 1)

So the offset is really #-22.  The destination, expressed relative to the
PC is PC+1-#22 or PC-#21
	#00405: #6edf	jump.3	PC-#21 (#003e4)

| Lastly, how does the program counter enter in to: PC+6+xxxx? 

I use PC to mean the address of the current instruction.  That is not the
way some processor manuals do it, but I find it simpler.

It is a pleasure to respond to such clearly stated questions.

alonzo