CBZ

Compare and branch on zero

This instruction compares the value in a register with zero, and conditionally branches to a label at a PC-relative offset if the comparison is equal. It provides a hint that this is not a subroutine call or return. This instruction does not affect condition flags.

Encoding: 19-bit signed PC-relative branch offset

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
	0	1	1	0	1	0	0
sf							op	imm19																			Rt

32-bit (sf == 0)

CBZ <Wt>, <label>

64-bit (sf == 1)

CBZ <Xt>, <label>

Decoding algorithm

constant integer t = UInt(Rt);
constant integer datasize = 32 << UInt(sf);
constant bits(64) offset = SignExtend(imm19:'00', 64);

Operation

constant boolean branch_conditional = TRUE;
constant bits(datasize) operand1 = X[t, datasize];
if IsZero(operand1) then
    BranchTo(PC64 + offset, BranchType_DIR, branch_conditional);
else
    BranchNotTaken(BranchType_DIR, branch_conditional);

Explanations

<Wt>: Is the 32-bit name of the general-purpose register to be tested, encoded in the "Rt" field.
<label>: Is the program label to be conditionally branched to. Its offset from the address of this instruction, in the range +/-1MB, is encoded as "imm19" times 4.
<Xt>: Is the 64-bit name of the general-purpose register to be tested, encoded in the "Rt" field.