Dynamic superscalar

(H&P pp.282)

• Dynamic scheduling should reduce dependence on sophisticated compiler and large no of registers
• Key issue: What hazards can block simultaneous issue?
  • Can issue dependent inst$^{\rm n}$s together
  • ... tags for dependent inst$^{\rm n}$s can be issued to reserv$^{\rm n}$ stations concurrently
  • But must check for structural stalls: all reserv$^{\rm n}$ stations full, or two inst$^{\rm n}$s need same one
  • Issue is in-order: if inst$^{\rm n}$_i of an n-word package has an issue stall, it's very hard to issue inst$^{\rm n}$_i+1-inst$^{\rm n}$_n
  • (n completions/clock needs n CDBs)
• Nasty problem:
  • Conditional branches