[sdcc-devel] [ sdcc-Feature Requests-927310 ] support for undoc'ed opcodes [mcs51]

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Feature Requests item #927310, was opened at 2004-04-01 07:14
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https://sourceforge.net/tracker/?func=detail&atid=350599&aid=927310&group_id=599

Category: None
Group: None
Status: Open
Priority: 1
Submitted By: Frieder Ferlemann (frief)
Assigned to: Nobody/Anonymous (nobody)
Summary: support for undoc'ed opcodes [mcs51]

Initial Comment:
Hi,

I recently came across some undocumented opcodes for the
8051. Unfortunately I feel I should rather not disclose 
where I got these from, nevertheless some (not all)
might be a valuable contribution to the SDCC compiler.


Among them are movc a,@SP and movc a,@SP++ 
"load acc with byte (indexed by the two topmost entries 
on the stack) from code memory". (Opcodes 0x04 and 0x14)
This f.e. allows to efficiently initialize variables 
from code memory:
	

	lcall	_LD_DATA32	
	.db	0xfe,0xfd,0xfc,0xfb


The lcall pushes the address of the beginning of the 
4 literal bytes on the stack so that a library routine 
can access them:


_LD_DATA32:
	movc 	a,@SP++		; .db 0x04
	mov	r0,a
	movc 	a,@SP++
	mov	r1,a
	movc 	a,@SP++
	mov	r2,a
	movc 	a,@SP++
	mov	r3,a
	ret			; return address is adjusted
	

As the 8051 instruction set is pretty much fully decoded,
an escape sequence enables these opcodes.
This is either the recently discussed "mov a,acc" (see
bug #916294)
https://sourceforge.net/tracker/?func=detail&atid=100599&aid=916294&group_id=599
or the instruction "acall ." (both of which are not
used in real
world applications). 
I tried both and got the expected results!


The movc opcodes documented above are not the only ones, 
though probably among the more useful. 
Here is a set of branch instructions, they seem to 
follow some 8051 symmetry and are mapped onto the
acall, ajmp opcodes:

Opcode	Mnemonic Description

0x01	bnt	branch not
0x11	bonb	branch on next branch
0x21	bob	branch on bug
0x31	bbw	branch both ways
0x41	brl	branch left
0x51	bfrm	branch from
0x61	baw	branch anywhere
0x71	binl	branch if nobody looks
0x81	bisl	branch if somebody looks
0x91	brnt	branch near to
0xA1	bbnv	branch beyond nirvana
0xB1	bib	branch if bored
0xC1	bew	branch either way
0xD1	brtl	branch to late
0xE1	bopb	branch on previous branch
0xF1	bsdm	branch seldom


There might be more!


Frieder


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