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Software => Software => Topic started by: mpuvdd on March 29, 2008, 09:38:37 AM

Title: $ + 1
Post by: mpuvdd on March 29, 2008, 09:38:37 AM
HI everyone,
I'm learning C and some assembly language currently, and was told that $ + 1 would delay 2 instruction cycles instead of 1.  Shouldn’t it just take 1 instruction cycle?  How's that work?  Also, $ does stand for PCL, right?

Title: Re: $ + 1
Post by: bulkhead on March 29, 2008, 10:40:19 PM
Which assembly language standard is this for?
Title: Re: $ + 1
Post by: mpuvdd on March 29, 2008, 11:44:19 PM
I'm not sure?  I'm using MPLAB IDE as a compiler, but that's all I know.
Title: Re: $ + 1
Post by: hazzer123 on March 30, 2008, 09:04:54 AM
I think that $ + 1 is shorthand for adding 1 to the PC register (program counter).

In a normal instruction, where the PC isnt modified, the PC automatically increments itself (which doesn't take a cycle). But when it's been modified, the MCU has to first jump to the new location (which takes a cycle) , before executing it. This adds the extra cycle. The $ + 1 just tricks the MCU into thinking the PC has been modified.

I think thats basically it.

Heres a webpage which goes into more detail (see the pipelining section) - (
Title: Re: $ + 1
Post by: izua on March 30, 2008, 01:35:16 PM
$ is a symbol reffering to current line. It's a shorthand for doing short jumps (although longer ones can be performed, too).
Less labels mean less clutter, so instead of doing a jump with "goto here", you can simply write "goto $+2".
it will not alter the PC in any different way than a goto does. It's just a different way of determining the jump location.