CODEWITHSUNDEEP
assemblymips
cpd1
cpd1

Reputation: 789

MIPS assembly statement - solution unclear

Reviewing some practice exam questions for class and one of the questions ask to write the assembly statements for a task...

Set the zero bit in the flag register

solution - add R1, R0, R0 ( there are many ways of doing it )

I'm not clear on why this would apply to the zero bit in the flag register?

Upvotes: 0

Views: 855

Answers (2)

Martin Rosenau
Martin Rosenau

Reputation: 18493

Real MIPS processors (normally) do not have "flags".

However some universitary MIPS variants (I found some in Google) add flags for educational purposes.

Unlike MIPS CPUs nearly all other CPU types do not have conditional jump instructions that can jump dependent on a register value (like "BLTZ"). Instead they have conditional jump instructions that jump dependent on the result of the previous arithmethic operation.

Therefore these CPUs must have a special "register" saving information about the last result (like "result was negative"). This register contains special bits - the so-called "flags". One of these bits is the "Zero Flag" that indicates that the result was zero.

Note:

  • The SPARC CPUs have an instruction set very similar to MIPS but they use a flag register instead of "BLTZ"-style relative jumps. So using such a CPU the original question would make sense even without adding non-standard registers.
  • I would use "add r0, r0, r0" because this would not modify the r1 register.

Upvotes: 2

Keith
Keith

Reputation: 338

The R0 register is hardwired to be the value zero, so adding it to itself will produce a zero result. Therefore the MIPS ALU will have its zero (Z) flag set.

Reference section 2.4.8.1 of the MIPS32 Architecture For Programmers document which states:

Two of the CPU general-purpose registers have assigned functions:

r0 is hard-wired to a value of zero, and can be used as the target register for any instruction whose result is to be discarded. r0 can also be used as a source when a zero value is needed.

Upvotes: 0

Related Questions