TBNZ

Test bit and branch if nonzero

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

Encoding: 14-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	1	1
b5							op	b40					imm14														Rt

TBNZ <R><t>, #<imm>, <label>

Decoding algorithm

constant integer t = UInt(Rt);

constant integer datasize = 32 << UInt(b5);
constant integer bit_pos = UInt(b5:b40);
constant bits(64) offset = SignExtend(imm14:'00', 64);

Operation

constant bits(datasize) operand = X[t, datasize];
constant boolean branch_conditional = TRUE;
if operand != '0' then
    BranchTo(PC64 + offset, BranchType_DIR, branch_conditional);
else
    BranchNotTaken(BranchType_DIR, branch_conditional);

Explanations

<R>: <t>: Is the number [0-30] of the general-purpose register to be tested or the name ZR (31), encoded in the "Rt" field.
<imm>: Is the bit number to be tested, in the range 0 to 63, encoded in "b5:b40".
<label>: Is the program label to be conditionally branched to. Its offset from the address of this instruction, in the range +/-32KB, is encoded as "imm14" times 4.