The success rate of branch prediction contributes significantly to the superscalar aspects of  z15 servers. This success is because the architecture rules prescribe that, for successful  parallel execution of an instruction stream, the correctly predicted result of the branch is  essential. The z15 branch prediction includes the following enhancements over z14: When a bad pointer is used or when code overlays a data area that contains a pointer to  code, a random branch is the result. This process causes a 0C1 or 0C4 abend. Random  branches are difficult to diagnose because clues about how the system got there are not  evident. With the wild branch hardware facility, the last address from which a successful branch  instruction was run is kept. z/OS uses this information with debugging aids, such as the  SLIP command, to determine from where a wild branch came. It can also collect data from that  storage location. This approach decreases the number of debugging steps that are  necessary when you want to know from where the branch came. 3.4.10 Wild branch GLYPH<SM590000> Branch prediction search pipeline extended from five to six cycles to accommodate new  predictors for increased accuracy/performance.  GLYPH<SM590000> Predictors enhancements: – SKOOT enhanced BTB search (SKip Over OffesetT entries remember where the  branch search should continue). This enhancement saves BTB1 access cycles while  searching for the next branch. – SSCRS (hardware-based super simple call-return stack). The major change is that it  does not necessarily have to return to the next sequential instruction (NSIA). This  feature supports return branches up to 8 bytes past the NSIA. – New TAGE (TAgged GEometric) history length branch predicator. GLYPH<SM590000> Branch prediction is simplified by removing the BTBp ‘‘BTB1 victim cache’’, which is  replaced by a write buffer. The two independent read ports were replaced by one  double-bandwidth port, which simplifies physical design significantly: – Level 1 Branch Target Buffer (BTB1): 2K rows x 4sets 2 K rows x 8sets  – Level 2 Branch Target Buffer (BTB2) size remains unchanged GLYPH<SM590000> Better power efficiency: Several structures were redesigned to maintain their accuracy  while less power is used through smart access algorithms. GLYPH<SM590000> New static IBM IA regions expanded from four to eight. To conserve space, prediction  structures do not store full target addresses. Instead, they use the locality and limited  ranges of “4gig regions” of virtual instruction addresses - IA(0:31). 110  IBM z15 (8561) Technical Guide