Compaq ECQD2KCTE Laptop User Manual


 
Instruction Descriptions 4–21
4.3.2 Unconditional Branch
Format:
Operation:
{update PC}
Ra PC
PC PC + {4*SEXT(disp)}
Exceptions:
Instruction mnemonics:
Qualifiers:
Description:
The PC of the following instruction (the updated PC) is written to register Ra and then the PC
is loaded with the target address.
The displacement is treated as a signed longword offset. This means it is shifted left two bits
(to address a longword boundary), sign-extended to 64 bits, and added to the updated PC to
form the target virtual address.
The unconditional branch instructions are PC-relative. The 21-bit signed displacement gives a
forward/backward branch distance of +/– 1M instructions.
PC-relative addressability can be established by:
BR Rx,L1
L1:
Notes:
BR and BSR do identical operations. They only differ in hints to possible branch-pre-
diction logic. BSR is predicted as a subroutine call (pushes the return address on a
branch-prediction stack), whereas BR is predicted as a branch (no push).
BxR Ra.wq,disp.al
!Branch format
None
BR Unconditional Branch
BSR Branch to Subroutine
None