Another mock exam Q - Convert machine code into instructions

[Spadez]

Hi,

Another question for you, sorry!

My question goes like this:
Convert the following machine code into an instruction: 0064h

This is my working out so far. I assume we want this in binary since figure 15.2 has the instructions in binary. I have this so far.

0064H
------
4 x 1 = .4
6 x 16 = .96
Total = .100

.100 in Binary is 0110 0100

What I dont understand is how to convert this binary into an instruction. There are 14 bits, and I have only 8. Bits 13 to 8 is the opcode apparently.

The way I see it is it has to be:
Bit Test f, Skip if Clear

This has the code:
01 10bb bfff ffff

Is this right? Im really not sure about this, and im guessing its probably wrong.

James

 

[Pommie]

The instruction that assembles to 0x0064 is CLRWDT.

Mike.

 

[Spadez]

Just to run this past again.

"Convert the following machine code into instructions"

b) 000h = 0000 0000 = NOP

c) 1283h = 0001 0010 1000 0011
I beleive this is a IORWF which has the following value:

00 0100 DFFF FFFF

This means D = 1 and F= 10 1000 0011


What does this actually mean?

 

[Spadez]

The binary code about is 16, but the Opcode is actually 14. Do I strip off the two MSB's? If that is the case then I think the answer will be BCF

01 0010 1000 0011

B = 101 = 5
F = 000 0011 = 3

I dont understand what these B and F values mean though.

 

[JimB]

"Convert the following machine code into instructions"

b) 000h = 0000 0000 = NOP

Correct

c) 1283h = 01 0010 1000 0011
I beleive this is a IORWF which has the following value:

00 0100 DFFF FFFF

00 0010 1000 0011 is IORWF STATUS, d

01 0010 1000 0011 is BCF STATUS, 05 where bit 05 is the RP0 bit.

This means D = 1 and F= 10 1000 0011

Make your mind up which opcode you are examining 0283 or 1283!

Wrong for the OP code first stated 1283h, there is no d bit.

And wrong for the second opcode 0283
D=1 and F= 000 0011 (3 in decimal)

What does this actually mean?

For all PIC instructions, when d=0 the result of the instruction is placed in the W register, and when d=1 the result is put into the register f.

To find which register is "3" look in the register file summary for your PIC, on the sheet I have beside me (16C84, no heckling from the gallery now!), "03" is the STATUS register.
(Just looked at the sheet for a 16F887 and 03 is the STATUS reg there, again, probably the same on all PICs).

JimB

Having re-hashed this reply, I think I have given a reasonable answer to a confused question.

 

[KeepItSimpleStupid]

Your HEX to binary need to be worked on because it's really simple:

0064H

is 0000 0000 1100 0100

Note that 0000 = 0, 0000 = 0, 1100 = 6h, and 0100 = 4h

So, just expand in groups of 4. It works in reverse too from binary to hex (separate into groups of 4).

 

[Spadez]

Thank you both for your reply. I understand now that my conversion was a bit ass about face, its a lot easier and quicker to do now.

Im sorry Jim, I was switching between the two accidently. It should be 1283h. So, with respect to that we have it like this:

01 0010 1000 0011
-- -- BB BFFF FFFF

So we have BBB as 101 (.5), and F as 000 0011 (.3)

Third 03H file register is Status and bit 5 of the status register is RP0. I understand im repeating what you said but it helps me to go through it. Thank you for helping me to clear it up.