MARIE Skipcond instruction

[Ed.R]

Hello everyone,

I've been trying to complete the HW4 using the MARIE simulator found on the textbooks website. http://computerscience.jbpub.com/ecoa/3e/

In my version of the textbook I have the following psuedo code for the Skipcond command.

if IR[11-10] = 00 then

    if AC < 0 then PC <- PC+1

else if IR[11-10] = 01 then

   if AC = 0 then PC <- PC+1

else if if IR[11-10] = 10 then

   if AC > 0 then PC <-PC + 1

So when I run the simulator and write my own assembly program I don't get results that are consistent with the textbook. For example my program uses Skipcond 001 and during execution the next instruction is skipped as if AC = 0 but in reality there is a negative integer value in AC. And when I use Skipcond 000 or Skipcond 010 I get equivalent behavior even though they should be exclusive operations.

Has anyone been using the simulator successfully? Is the ISA different in more recent versions of the textbook?

Thanks.

-Eddie

[Will Zegers]

I was having the same issue trying to code exercise #26 and finally found some clarification online. Looks like the book instructions may be wrong and the skipcond should like

  Skipcond 800 if AC > 0,
  Skipcond 400 if AC = 0,
  Skipcond 000 if AC < 0

so using "800" instead of "400" finally got my code working right. Hope that helps.

Will

[Will Zegers]

On a side note: I noticed also that you can put pretty much any number with skipcond and it won't give an assembly error, but when it runs it will just treat it like a NOP. Makes debugging code frustrating if you don't know about it.

[Eddie R]

Thanks Will, that really cleared things up!

[Frank (sjsu) Lin]

very nice collaboration and good work.

[zgfn1]

actually, the book is not wrong.

In binary, 

IR[11-10] = 00 means 00xx xxxx xxxx, and you can write 0xx - 3xx in hexdecimal

IR[11-10] = 01 means 01xx xxxx xxxx, and you can write 4xx - 6xx in hexdecimal

IR[11-10] = 10 means 10xx xxxx xxxx, and you can write 8xx - Bxx in hexdecimal

Therefore, we can write Skipcond 000, Skipcond 400, and Skipcond 800.

在 2012年10月22日星期一UTC+11上午11时48分03秒，Will Zegers写道：