Re: [f-cpu] Reamrks and suggestions about the manual

[Michael Riepe <michael@stud.uni-hannover.de>]

On Sun, Jul 07, 2002 at 02:36:01PM +0200, Thomas Lavergne wrote:
> Here's some remarks about the ISA decritpion in the manual, some are 
> little write error, other are remarks about coding of the ISA...
> 
> Page 94
> -------
> 
> You have these two form defined at the start :
>    addsubs r3, r2, r1
>    saddsubs r3, r2, r1
> 
> but no explication of the s suffixe later

That might mean `saturated'.

[...]
> Page 139
> --------
> 
> You don't explain the encoding of the flags xtcs, I suppose it was the 
> same than for the bitop operand but it could be could to say it.
> 
> At the top of the page you say 'logici.xxxx' so I understand this style 
> of codding 'logici.0111' like for the logic operand but ater you say 
> 'logici.s' like for bitop, we must clarify this and I think it was best 
> to have the same form for logic and logici even if we don't have all the 
> function of logic in logici.

The `logic[i].xxxx' instruction is obsolete. And it's rather hard to
grok right because the function encoding is non-intuitive - e.g. does
`logic.0010 A, B, C' mean `A and not B' or `B and not A'? Is `logic.0001'
an AND or NOR operation? Both variants are possible, depending on the
interpretation of the function field.

ROP2 instructions and bitop use the same function encoding for the
AND/ANDN/XOR/OR functions (using bits stolen from the 8-bit opcode field).
According to the VHDL source, the encoding is

	000 - and  (ROP2/bitop)
	001 - andn (ROP2/bitop)
	010 - xor  (ROP2/bitop)
	011 - or   (ROP2/bitop)
	100 - nor  (ROP2 only)
	101 - xnor (ROP2 only)
	110 - orn  (ROP2 only)
	111 - nand (ROP2 only)

[...]
> Page 184
> --------
> 
> loopentry use a non standard form (op[8], 0[18], r1[6])

`loopentry r1' is a special case; it's equivalent to `loadaddri 0, r1'.
That is, the encoding is (op[8], flags[2], imm[16], r1[6]) with an
immediate of zero and both flags cleared (D=0 because it's not a data
address, S=0 because the immediate is not negative).

-- 
 Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
 "All I wanna do is have a little fun before I die"
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