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3–12 Alpha Architecture Handbook
3.3.1.2 Memory Format Jump Instructions
For computed branch instructions (CALL, RET, JMP, JSR_COROUTINE) the displacement
field is used to provide branch-prediction hints as described in Section 4.3.
3.3.2 Branch Instruction Format
The Branch format is used for conditional branch instructions and for PC-relative subroutine
jumps. It has the format shown in Figure 3–3.
Figure 3–3: Branch Instruction Format
A Branch format instruction contains a 6-bit opcode field, one 5-bit register address field (Ra),
and a 21-bit signed displacement field.
The displacement is treated as a 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. Overflow is ignored in this calculation. The target virtual address
(va) is computed as follows:
va ← PC + {4*SEXT(Branch_disp)}
3.3.3 Operate Instruction Format
The Operate format is used for instructions that perform integer register to integer register
operations. The Operate format allows the specification of one destination operand and two
source operands. One of the source operands can be a literal constant. The Operate format in
Figure 3–4 shows the two cases when bit <12> of the instruction is 0 and 1.
Figure 3–4: Operate Instruction Format
031 26 25 2120
Opcode Ra Branch_disp
031 26 25
0
1312 112120 16 15 5 4
Opcode Ra Rb
SBZ
Function Rc
031 26 25
1
1312 112120 5 4
Opcode Ra LIT Function Rc