Section 2.7 – Making Decisions & Branches in LEGv8

1. Why Decision Instructions Exist

• A computer differs from a calculator because it can choose different instruction sequences depending on computed values.
• 1947 Burks, Goldstine, von Neumann quote: introduced the idea of conditional transfer instructions based on the sign of a number (zero treated as plus).
• High-level languages: if, else, goto.
• In LEGv8, decisions compile to conditional branches.

2. Two Basic Conditional Branches in LEGv8

• CBZ reg, Label – “Compare and Branch if Zero”

  Jumps to Label if register = 0.

• CBNZ reg, Label – “Compare and Branch if Not Zero”

  Jumps to Label if register ≠ 0.

3. Compiling an if–then–else Statement

C code:

if (i == j)
  f = g + h;
else
  f = g - h;

Assembly version:

SUB X9, X22, X23      // X9 = i - j
CBNZ X9, Else         // go to Else if i ≠ j (X9 ≠ 0)
ADD X19, X20, X21     // f = g + h
B Exit                // skip Else
Else: SUB X19, X20, X21 // f = g - h
Exit:

• If i == j: executes ADD, then branches to Exit, skipping subtract.
• If i ≠ j: skips ADD, executes SUB in Else block.
• Both paths converge at Exit.