assemblyavr
For example on 8-bit AVR given a register r16 I do something like ldi r16, 44 and this way I can give it positive values from 0 to 255.
But if I want negative clearly in two's complement I can get a range of -128…+127 but I can't do that like.
ldi r16, -13
And furthermore how does the microprocessor know whether I want all 8-bits for positive 0…255 or negative -128 to 127? Thank you very much for your help.
The AVR libraries don't compile in support for printing of floating point values by default. This is to save space on their limited resource environments. You need to tell the compiler and the linker to enable support for this feature.
From the avr-libc documentation on vfprintf():
Since the full implementation of all the mentioned features becomes fairly large, three different flavours of
vfprintf()can be selected using linker options. The defaultvfprintf()implements all the mentioned functionality except floating point conversions. A minimized version ofvfprintf()is available that only implements the very basic integer and string conversion facilities, but only the # additional option can be specified using conversion flags (these flags are parsed correctly from the format specification, but then simply ignored). This version can be requested using the following compiler options:
-Wl,-u,vfprintf -lprintf_minIf the full functionality including the floating point conversions is required, the following options should be used:
-Wl,-u,vfprintf -lprintf_flt -lm
(emphasis mine)
Since you are using CodeVisionAVR as we discussed in chat, they have this option built in to their configuration window as seen on Page 4, Step 9 of this tutorial:
The instruction you need has the mnemonic CPI (compare with immediate).
This will set or clear a flag depending on the result.
You can then branch after checking the flag.
Have a look at the following example. It's from the [8-bit] AVR instruction summary (http://www.atmel.com/images/doc0856.pdf).
The example below is from page 24:
Example:
cpi r20,5 ; Compare r20 to the value 5
brbc 1,noteq ; Branch if Zero Flag cleared
...
noteq:nop ; Branch destination (do nothing)
There are other instructions that will do slightly different comparisons (like compare two registers, instead of comparing a register with a literal).
Best Answer
The processor doesn't know and doesn't care.
The advantage of using two's complement representation for signed numbers is that add and subtract operations produce the same results from the same inputs whether you say the inputs are unsigned or two's complement values.
Of course you will have to watch out for different overflow conditions in the two cases --- but typically the processor provides flags that can detect overflow for either type of operation.
If you can't, that's a limitation of your assembler, not of the processor.
If you use the Microchip AVR Assembler, you should be able to exactly that, according to the documentation.