Post by thread:[PIC] HALT instruction

'[PIC] HALT instruction'

I came across some code which included the HALT instruction, and can't

find any documentation as to what it is.  MPASM accepts the

instruction, and generates an opcode for it for both PIC16 and PIC18

families.  MPLAB, however, doesn't disassemble it in the program

memory window.  Does anyone know what it's for?

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John W. Temples, III

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At 11:06 AM 10/4/2004, spam_OUTpiclistTakeThisOuTxargs.com wrote:

>I came across some code which included the HALT instruction, and can't

>find any documentation as to what it is.  MPASM accepts the

>instruction, and generates an opcode for it for both PIC16 and PIC18

>families.  MPLAB, however, doesn't disassemble it in the program

>memory window.  Does anyone know what it's for?

Any chance that the op-code generated looks anything like:

     goto  $

All I can think of at the moment.

dwayne

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At 12:06 PM 10/4/2004, piclistKILLspamxargs.com wrote:

>I came across some code which included the HALT instruction, and can't

>find any documentation as to what it is.  MPASM accepts the

>instruction, and generates an opcode for it for both PIC16 and PIC18

>families.  MPLAB, however, doesn't disassemble it in the program

>memory window.  Does anyone know what it's for?

Not documented in the proc's data sheet?

Normally, halt is NOT a "jump here".

Normally, it is a way to reduce CPU consumption by halting operations in the ALU.

I/O normally continues, so that you have some way to exit the halt, like on an interrupt.

There are so may variations, it's hard to be too specific.

Halt, Sleep, Snooze.... :)

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I think that John refers to the HALT instruction that is recognized by the

MPASM compiler. I've tested a few minutes ago and I don't see what it

does... Might it be an instruction for the ICE or ICD or anything like that?

The help says that it stop execution, but I thnk it's refering to the F5 -

Halt...

Mauricio Jancic

Janso Desarrollos

Microchip Consultant Program Member

(54) 11-4542-3519

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>>{Original Message removed}

>>Any chance that the op-code generated looks anything like:

>>

>>     goto  $

No, It generate a 0x0061 opcode, which has no translation to "human code" or

assembler... Try it your self in the MPLAB.

Mauricio Jancic

Janso Desarrollos

Microchip Consultant Program Member

(54) 11-4542-3519

EraseMEinfospam_OUTTakeThisOuTjanso.com.ar

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At 01:02 PM 10/4/2004, Mauricio Jancic wrote:

>I think that John refers to the HALT instruction that is recognized by the

>MPASM compiler. I've tested a few minutes ago and I don't see what it

>does... Might it be an instruction for the ICE or ICD or anything like that?

>The help says that it stop execution, but I thnk it's refering to the F5 -

>Halt...

Compiler directive instead of processor instruction?

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----- Original Message ----- 

From: "Mauricio Jancic" <infospam_OUTjanso.com.ar>

Subject: RE: [PIC] HALT instruction

> No, It generate a 0x0061 opcode, which has no translation to "human code"

or

> assembler... Try it your self in the MPLAB.

Interesting, though ... other nearby opcodes:

  62 option

  63 sleep

  64 clrwdt

certainly sounds like a control sort of instruction.

--McD

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On Mon, 2004-10-04 at 14:26, John J. McDonough wrote:

> ----- Original Message ----- 

> From: "Mauricio Jancic" <@spam@infoKILLspamjanso.com.ar>

> Subject: RE: [PIC] HALT instruction

> 

> 

> > No, It generate a 0x0061 opcode, which has no translation to "human code"

> or

> > assembler... Try it your self in the MPLAB.

> 

> Interesting, though ... other nearby opcodes:

>   62 option

>   63 sleep

>   64 clrwdt

> certainly sounds like a control sort of instruction.

Could it be a breakpoint sort of deal for the ICD2? TTYL

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John J. McDonough wrote:

> Interesting, though ... other nearby opcodes:

>   62 option

>   63 sleep

>   64 clrwdt

> certainly sounds like a control sort of instruction.

My sentiments too.

Perhaps a 'breakpoint' sleep instruction?

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At 02:32 PM 10/4/2004 -0400, you wrote:

>On Mon, 2004-10-04 at 14:26, John J. McDonough wrote:

> > {Original Message removed}
On Mon, 4 Oct 2004, Dwayne Reid wrote:

> Any chance that the op-code generated looks anything like:

>

>      goto  $

No, it's not an opcode that MPLAB can disassemble.  On the PIC18, it

generates a 0000 0000 0000 0001 opcode, which is not documented in the

18 family datasheet.  It appears to do nothing in the simulator.

--

John W. Temples, III

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> It [the HALT instruction] appears to do nothing in the simulator.

The next step usualy is to try the following command :

  "HALT ! Or I'll pull the trigger !"

:-)

Jan-Erik.

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John Temples wrote:

> Does anyone know what it's for?

Hi John,

I can't answer your question directly but you might want to check this out

(it's a snippet from a past post I made on the PICList):

There was a past discussion on the Microchip forum regarding undocumented

instructions (i.e. TRAP) that are decoded by MPLAB.

One of the Microchip moderator's posted this:

=====

One trick I revealed at a MASTERs conference was that you can assemble the

TRAP instruction into your code to get more than one breakpoint with the

MPLAB ICD 2. It's a little more laborious, because you've got to insert it

into your code and assemble, but when you absolutely must have more

breakpoints, this is a nice trick.

Of course, you can manually insert a TRAP into program memory, but you will

overwrite the instruction at that address.

dj

====

Best regards,

Ken Pergola

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On Mon, 4 Oct 2004, Spehro Pefhany wrote:

> The help file says:

>

> "HaltHalt

> A stop of program execution. Executing Halt is the same as stopping at a

> breakpoint."

That's referring to the MPLAB halt; i.e., pressing F5 in debug mode.

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The halt instruction sounds interesting. I don't see application for it in

a typical embedded system. I have sometimes done a goto $ to force a

watchdog timeout to reset the system. On TRAP, and the need to overwrite a

code location when putting it in as a breakpoint, I imagine this is what

MPLAB does. When you remove the breakpoint, it probably just writes the

original instruction back. My first microprocessor experience was with the

MC6802. It had several interrupts including a nonmaskable, a maskable, and

a "software interrupt." The SWI (0x3f) was like any other interrupt except

that it actually took a byte of code space. It would stick the registers

on the stack, then vector to the address listed in the table at the top of

memory (something like at 0xfff8). I wrote a monitor program that would

dump processor status (PC, A, B, X, flags, SP) to my terminal (Lear

Siegler ADM-1) when the program hit a location with 0x3f in it. I'd guess

that the ICD and MPLAB do something similar.

I'd be interested in finding out what the halt instruction does, though.

Harold

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Jan-Erik Soderholm wrote:

> 

> > It [the HALT instruction] appears to do nothing in the simulator.

> 

> The next step usualy is to try the following command :

> 

>   "HALT ! Or I'll pull the trigger !"

> 

> :-)

No no no... You want the "HCF" instruction from

the old Motorola (now Freescale) 6800 data sheet.

http://computing-dictionary.thefreedictionary.com/Halt%20and%20Catch%20Fire

Halt and Catch Fire - (HCF) Any of several undocumented and 

semi-mythical machine instructions with destructive side-effects, 

supposedly included for test purposes on several well-known architectures

going as far back as the IBM 360. The Motorola 6800 microprocessor 

was the first for which an HCF opcode became widely known. 

This instruction caused the processor to toggle a subset 

of the bus lines as rapidly as it could;

 in some configurations this could actually cause lines to burn up.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

So much for 'mythical'....

R

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According to Microchip Tech support:

This instruction is used by emulators and debuggers for breakpoint

operation. It isn't a user instruction. 

I've just requested a full list of instructions and explanation to see if

there is something usefull...

regards

Mauricio Jancic

Janso Desarrollos

Microchip Consultant Program Member

(54) 11-4542-3519

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I don't know about you guys - but I find that undocumented bits like

these actually make languages/code/products/etc a whole lot more

interesting and/or useful.

I know it's a very simple example - but cast your minds back to MS-DOS's

FORMAT and the /AUTOTEST switch. This'll format a disk without asking if

you really want to do it. Useful? Probably not. But it just goes to show

that when you thought that you knew something inside-out - there are

still hidden bits in there to be investigated to re-spark your interest.

Why is it there? Who put it there? Is there a story behind it? And so

on...

Speaking of which - does anyone know of any other undocumented PIC, (or

other), instructions? (Myths or otherwise!)

Cheers,

Stuart

{Original Message removed}