Reality Signal Processor/CPU Core

Scalar unit (SU)

The scalar is the half of the RSP core that is similar to a standard MIPS R4000 32-bit CPU. It has 32 32-bit registers (conventionally called r0-r31) and implement most standard opcodes. This page does not describe the whole scalar unit as standard MIPS documentation suffices, but it highlights the main difference.

Missing opcodes

The following opcodes are not implemented by RSP:

• Multiplication units. RSP does not have a multiplication unit so there is no MULT, MULTU, DIV, DIVU, MFHI, MFLO, MTHI, MTLO.
• 64-bit instructions. RSP only has 32-bit scalar registers in SU, so there is no 64-bit opcodes (the ones starting with D such as DADDIU, DSRL, etc.) nor 64-bit memory accesses such as LD, SD, LDL, SDL.
• No opcodes for misaligned memory accesses. All memory accesses to DMEM can be correctly performed also to misaligned addresses, using the standard opcodes like LW / SW or LH / LHU / SH, so there is no LWL, LWR, SWL, SWR.
• No traps or exceptions. RSP does not implement any form of interrupt or exception handling, so there is no SYSCALL nor trap instructions (TGE, TLT, etc.). BREAK is available but it has a special behavior (see below).
• No support for likely branches. The "likely" variant of all branches is not supported. The missing opcodes are the ones ending with L (such as BEQL, BLEZL, etc.)

Memory access

RSP is a harvard architecture. All opcodes are fetched from IMEM (4KB) and all data is access in DMEM (4KB).

The PC register is 12-bit. All higher address bits in branch / call instructions are thus ignored. When PC reaches the last opcode (at 0xFFC), execution continues to the first opcode in IMEM (PC wraps to 0x000).

All accesses to DMEM are performed using the lowest 12 bits of the address calculated by the load/store instruction (higher bits are ignored). Moreover, contrary to standard MIPS architecture, the RSP can correctly perform misaligned memory accesses (eg: it is possibly to fetch a 32-bit word at address 0x001, that will contain the 4 bytes at 0x1-0x5). Standard MIPS architecture allows to do misaligned addresses only using the LWL/LWR or SWL/SWR couples, which are not required on the RSP.

Vector Unit (VU)

The VU is the internal unit of the RSP CPU core that is able to perform fixed-point SIMD calculations. It is a proprietary design which does not follow any standard specification. Its opcodes and registers are exposed to the core via the COP2 interface.

Vector registers and glossary

VU contains 32 128-bit SIMD registers, each organized in 8 lanes of 16-bit each one. Most VU opcodes perform the same operation in parallel on each of the 8 lanes. The arrangement is thus similar to x86 SSE2 registers in EPI16 format.

The vector registers array is called VPR in this document, so VPR[4] refers to the fifth register (usually called v4 in assembly). When referring to specific portions of the register, we use the following convention:

• VPR[vt][4..7] refers to byte indices, that is bytes from 4 to 7, counting from the higher part of the register (in big-endian order).
• VPR[vt]<4..7> refers to specific lane indices, that is lanes from 4 to 7 counting from the higher part of the register (in big-endian order).
• Within each lane, VPR[vt]<2>(3..0) refers to inclusive bit ranges. Notice that bits are counted as usual in little-endian order (bit 0 is the lowest, bit 15 is the highest), and thus they are written as (high..low).

Ranges are specified using the beg..end inclusive notation (that is, both beg and end are part of the range).

The concatenation of disjoint ranges is written with a ,, for instance: [0..3,8..11] means 8 bytes formed by concatenating 4 bytes starting at 0 with 4 bytes starting at 8.

Vector lanes are usually interpreted as a fixed point number. As a homebrew programmer, it is useful to understand the meaning of each opcode and its correct usage while writing code, which goes beyond the exact hardware description of how bits are shuffled around. To refer to a fixed point number, we use the syntax S1.15 where "S" means "signed" (while "U" is "unsigned"), "1" is the number of bits for the integral part, and "15" are the number of bits for the fractional part.

Accumulator

The RSP contains a 8-lane SIMD accumulator, that is used implicitly by multiplication opcodes. Each of the 8 lanes is 48-bits wide, that allows to accumulate intermediate results of calculations without the loss of precision that would incur when storing them into a 16-bit lane in a vector register.

It is possible to extract the contents of the accumulator through the VSAR opcode; one call to this opcode can extract a 16-bit portion of each lane and store it into the specified vector register. The three portions are conventionally called ACCUM_LO (bits 15..0 of each lane), ACCUM_MD (bits 31..16 of each lane), and ACCUM_HI (bits 47..32 of each lane).

If you exclude the VSAR instruction that cuts the accumulator piecewise for extracting it, it is better to think of it a single register where each lane is 48-bits wide.

Control registers

The VU contains 3 16-bit control registers: VCC, VCO, VCE.

These registers are used as flag registers by several opcodes. As with most flags, even though they have a general meaning that is generally valid, they tend to also be used in some mind-twisting way to obtain the desired result. It doesn't really make sense to try to describe them at the general level and instead each instruction will explain if/how it uses or modifies the control registers.

To read/write the contents of the control registers, the ctc2 / cfc2 instructions can be used.

Clamping

Multiplication opcodes perform a clamping step when extracting the accumulator into a vector register. Notice that each lane of the accumulator is always treated as a signed 48-bit number.

This is the pseudo-code for signed clamping (no surprises):

function clamp_signed(accum)
    if accum < -32768  => return -32768
    if accum > 32767   => return 32767
    return accum

The returned value is thus always within the signed 16-bit range.

This is the pseudo-code for unsigned clamping:

function clamp_unsigned(accum)
    if accum < 0       => return 0
    if accum > 32767   => return 65535
    return accum

Notice that in unsigned clamping, the saturating threshold is 15-bit, but the saturated value is 16-bit.

Element field

Most VU instructions have a 3-register format with an additional modifier called "element field". For instance (using GNU assembly syntax):

opcode v0, v1, v2,e(7)

e(7) is the "element modifier". Normally (and especially in GNU syntax, which is more orthogonal and uniform), it refers to a specific lane of the third register, which is why it is common to format it without a leading whitespace. In this example, it "selects" lane 7 of register v2. The exact meaning of the element modifier varies for different instruction groups, and also the way it is assembled changes wildly. Pay attention to the description of each instruction group to check what the element modifier means and how it is encoded in the opcode.

Broadcast modifier

One of the most common uses of the element field is the broadcast modifier. This modifier is used by computational instructions and select instructions and allows to "broadcast" (duplicate) one or more lanes to other lanes, just for the purpose of the current opcode. For instance:

vaddc $v01, $v04,e(1)

e(1) is the broadcast modifier. Normally, the instruction would add the two registers lane by lane; with the modifier, the second lane (index 1) of $v04 is added to all lanes of $v01.

The modifier is stored in the element field of the opcode

element	GNU syntax	SGI syntax	Lanes being accessed	Description
0			0,1,2,3,4,5,6,7	Normal register access (no broadcast)
1			0,1,2,3,4,5,6,7	Normal register access (no broadcast)
2	e(0q)	[0q]	0,0,2,2,4,4,6,6	Broadcast 4 of 8 lanes
3	e(1q)	[1q]	1,1,3,3,5,5,7,7	Broadcast 4 of 8 lanes
4	e(0h)	[0h]	0,0,0,0,4,4,4,4	Broadcast 2 of 8 lanes
5	e(1h)	[1h]	1,1,1,1,5,5,5,5	Broadcast 2 of 8 lanes
6	e(2h)	[2h]	2,2,2,2,6,6,6,6	Broadcast 2 of 8 lanes
7	e(3h)	[3h]	3,3,3,3,7,7,7,7	Broadcast 2 of 8 lanes
8	e(0)	[0]	0,0,0,0,0,0,0,0	Broadcast single lane
9	e(1)	[1]	1,1,1,1,1,1,1,1	Broadcast single lane
10	e(2)	[2]	2,2,2,2,2,2,2,2	Broadcast single lane
11	e(3)	[3]	3,3,3,3,3,3,3,3	Broadcast single lane
12	e(4)	[4]	4,4,4,4,4,4,4,4	Broadcast single lane
13	e(5)	[5]	5,5,5,5,5,5,5,5	Broadcast single lane
14	e(6)	[6]	6,6,6,6,6,6,6,6	Broadcast single lane
15	e(7)	[7]	7,7,7,7,7,7,7,7	Broadcast single lane

Instructions overview

Loads and stores

31..26	25..21	20..16	15..11	10..7	6..0
LWC2 or SWC2	base	vt	opcode	element	offset

The instructions perform a load/store from DMEM into/from a vector register.

• base is the index of a scalar register used as base for the memory access
• offset is a signed offset added to the value of the base register (with some scaling, depending on the actual instruction).
• vt is the vector register.
• element is used to index a specific byte/word within the vector register, usually specifying the first element affected by the operation (thus allows to access sub-portions of the vector register).

List of all loads and stores opcodes

Group	Opcode	Instruction	Description
Scalar	0x00	lbv / sbv	Load / Store 1 byte into/from a VPR
	0x01	lsv / ssv	Load / Store 2 bytes into/from a VPR
	0x02	llv / slv	Load / Store 4 bytes into/from a VPR
	0x03	ldv / sdv	Load / Store 8 bytes into/from a VPR
128-bit	0x04	lqv	Load (up to) 16 bytes into a VPR, left-aligned
	0x05	lrv	Load (up to) 16 bytes into a VPR, right-aligned
	0x04	sqv	Store (up to) 16 bytes from a VPR, left-aligned
	0x05	srv	Store (up to) 16 bytes from a VPR, right-aligned
8-bit packed	0x06	lpv / spv	Load / store 8 8-bit signed values into a VPR
	0x07	luv / suv	Load / store 8 8-bit unsigned values into a VPR
	0x08	lhv / shv	Load / store 8 8-bit unsigned values into VPR, accessing every other byte in memory
	0x09	lfv / sfv	Load / store 4 8-bit unsigned values into VPR, accessing every fourth bytes in memory
Transpose	0x01	swv
	0x0B	ltv	Load 8 lanes from 8 GPRs into a VPR
	0x0B	stv	Store 8 lanes of a VPR into 8 GPRs

Single-lane instructions

31..26	25	24..21	20..16	15..11	10..6	5..0
COP2	1	vt_elem	vt	vd_elem	vd	opcode

Single-lane instructions are an instruction group that perform operations on a single lange of a single input register (VT<se>), and store the result into a single lane of a single output register (VD<de>).

Example syntax:

vmov $v01, e(4), $v05, e(6)

In this example, the value in lane $v05<6> is moved to lane $v01<4>. In the assembly syntax, the broadcast modifier syntax is used, but no actual broadcast is performed, as the instructions operate on the single specified lane. Only the single-lane broadcast modifiers (e(0) ... e(7)) are supported.+

In the opcode, the fields vt_elem and vd_elem are used to compute se and de that is to specify which lane, respectively of the input and output register, is affected.

vd_elem is 5 bits long (range 0..31); the highest bits are always ignored, and the destination lane de is simply vd_elem(2..0).

vt_elem is 4 bits long (range 0..15). When vt_elem(3) is 1, vt_elem(2..0) is actually used as source lane se, as expected. When vt_elem(3) is 0, a hardware bug is triggered and portions of the lower bits of vt_elem are replaced with portion of the bits of vd_elem while computing se. Specifically, all bits in vt_elem from the topmost set bit and higher are replaced with the same-position bits in vd_elem. Notice that this behaviour is actually consistent with what happens when vt_elem(3) is 1, which means that there is no need to think of it as a special-case. Pseudo-code:

de(2..0) = vd_elem(2..0)
msb = highest_set_bit(vt_elem)
se(2..0) = vd_elem(2..msb) || vt_elem(msb-1..0)

Single-lane instructions

Opcode	Instruction	Description
0x33	vmov	Copy one lane of a VPR into another VPR
0x30	vrcp	Compute the 32-bit reciprocal of a 16-bit fixed point
0x34	vrsq	Compute the 32-bit reciprocal square root of a 16-bit fixed point
0x32	vrcph	Extract the higher 16-bit of the result of a previous VRCP
0x36	vrsqh	Extract the higher 16-bit of the result of a previous VRSQ
0x31	vrcpl	Compute the 32-bit reciprocal of a 32-bit fixed point
0x33	vrsql	Compute the 32-bit reciprocal square root of a 32-bit fixed point
0x37	vnop	No operation (?)
0x3F	vnull	No operation (?)

Computational instructions

31..26	25	24..21	20..16	15..11	10..6	5..0
COP2	1	element	vt	vs	vd	opcode

Instructions have this general format:

VINSN vd, vs, vt,e(…)

where e(…) is the broadcast modifier (as found in other SIMD architectures), that modifies the access to vt duplicating some lanes and hiding others.

This is the list of opcodes in this group.

Opcode	Instruction	Description
0x00	vmulf	Vector multiply S1.15 * S1.15, with rounding and signed clamping
0x01	vmulu	Vector multiply S1.15 * S1.15 with rounding and unsigned clamping
0x04	vmudl	Vector multiply U0.16 * U0.16 with signed clamping
0x05	vmudm	Vector multiply S0.16 * U0.16 with signed clamping
0x06	vmudn	Vector multiply U0.16 * S0.16 with signed clamping
0x07	vmudh	Vector multiply S0.16 * S0.16 with signed clamping
0x08	vmacf	Like VMULF, but also add the result to the accumulator
0x09	vmacu	Like VMULU, but also add the result to the accumulator
0x0C	vmadl	Like VMUDL, but also add the result to the accumulator
0x0D	vmadm	Like VMUDM, but also add the result to the accumulator
0x0E	vmadn	Like VMUDN, but also add the result to the accumulator
0x0F	vmadh	Like VMUDH, but also add the result to the accumulator
0x10	vadd	Vector add with carry
0x13	vabs	Vector absolute value
0x14	vaddc	Vector add writing overflow into carry
0x1D	vsar	Read a portion of the accumulator into a VPR
0x28	vand	Vector bitwise and (a & b)
0x29	vnand	Vector bitwise nand (~(a & b))
0x2A	vor	Vector bitwise or (a | b)
0x2B	vnor	Vector bitwise nor (~(a | b))
0x2C	vxor	Vector bitwise xor (a ^ b)
0x2D	vnxor	Vector bitwise nxor (~(a ^ b))

Select instructions

31..26	25	24..21	20..16	15..11	10..6	5..0
COP2	1	element	vt	vs	vd	opcode

Instructions have this general format:

VINSN vd, vs, vt, e(…)

where e(…) is the broadcast modifier (as found in other SIMD architectures), that modifies the access to vt duplicating some lanes and hiding others. See the Computational instructions section for details.

This is the list of opcodes in this group:

Opcode	Instruction	Description
0x20	vlt	Select the lower value between two VPR
0x21	veq	Compare two VPR to check if they are equal
0x22	vne	Compare two VPR to check if they are different
0x23	vge	Select the greater or equal value between two VPR
0x24	vcl	Clip a VPR against two bounds (lower 16-bits)
0x25	vch	Clip a VPR against two bounds (higher 16-bits)
0x26	vcr	Clip a VPR against a pow-2 bound
0x27	vmrg	Merge two VPR selecting each lane according to flags

VU/SU Moves

31..26	25..21	20..16	15..11	10..7	6..0
COP2	opcode	rt	vs	vs_elem	0

These are the standard MIPS opcodes for moving data in/out the coprocessor registers.

opcode	Instruction	Description
0x0	mfc2	Copy a lane of a VPR into a GPR
0x2	cfc2	Copy a VU control register into a GPR
0x4	mtc2	Copy a GPR into a lane of a VPR
0x6	ctc2	Copy a GPR into a VU control register

Vector moves follow the same format as standard MIPS coprocessor moves, but use part of the lower 11 bits (which are normally unused) to specify the element field, selecting which lane of the VPR is accessed. Notice that, vs_elem in this case is not a broadcast modifier: it specifies a byte offset (not a lane index!), so to copy a lane, lane*2 must be specified.

This is an example using GNU syntax:

mtc2 a1, $v04,e(4)

This example will copy the lower 16 bits of GPR a1 into the fifth lane of $v04. This opcode is assembled with vs_elem = 8, as explained above.

mtc2 moves the lower 16 bits of the general purpose register rt to the bytes VS[vs_elem+1..vs_elem]. If vs_elem is 15, only VS[vs_elem] is written (with rt[15..8]).

mfc2 moves the 2 bytes VS[vs_elem+1..vs_elem] to GPR rt, sign extending the 16 bits value to 32 bits. If vs_elem is 15, the lower byte is taken from byte 0 of the register (that is, it wraps around).

ctc2 moves the lower 16 bits of GPR rt into the control register specified by vs, while cfc2 does the reverse, moving the control register specified by vs into GPR rt, sign extending to 32 bits. Note that both ctc2 and cfc2 ignore the vs_elem field. For these instructions, the control register is specified as follows:

vs	Register
0	VCO
1	VCC
2	VCE

Instruction details

Scalar loads: LBV, LSV, LLV, LDV

31..26	25..21	20..16	15..11	10..7	6..0
LWC2	base	vt	opcode	element	offset

Assembly

lsv $v01,e(2), 0,s0    ; Load the 16-bit word at s0 into the third lane of $v01
lbv $v04,8, 0,s1       ; Load the 8-bit word at s1 into the 9th byte of $v04 (MSB of lane 4)

Notice that it is possible to specify the lane syntax for the element field to refer to a specific lane, but if the access is made using llv or ldv (4 or 8 bytes), it will overflow into the following lanes.

Pseudo-code

addr = GPR[base] + offset * access_size
data = DMEM[addr..addr+access_size-1]
VPR[vt][element..element+access_size-1] = data

Description

These instructions load a scalar value (1, 2, 4, or 8 bytes) from DMEM into a VPR. Loads affect only a portion of the vector register (which is 128-bit); other bytes in the register are not modified.

The address in DMEM where the value is fetched is computed as GPR[base] + (offset * access_size), where access_size is the number of bytes being accessed (eg: 4 for llv). The address can be misaligned: despite how memory accesses usually work on MIPS, these instructions perform unaligned memory accesses.

The part of the vector register being accessed is VPR[vt][element..element+access_size], that is element selects the first accessed byte within the vector register. When element+access_size is bigger than 15, fewer bytes are processed (eg: llv with element=13 only loads 3 byte from memory into VPR[vt][13..15]).

Usage

These instructions are seldom used. Normally, it is better to structure RSP code to work across full vectors to maximize parallelism. Input data should already be provided in vectorized format by the CPU, so that it is possible to use a vector load (lqv, in case the input is made of 16-bit data) or a packed load (luv/lpv, in case the input is made of 8-bit data). Consider also using mtc2 to load a 16-bit value into a lane of a VPR when the value is available in a GPR.

A possible use-case for these instructions is to reverse the order of the lanes. For instance, in audio codecs, windowing algorithms often work combining sequences audio samples with other sequences in reverse order. RSP does not have an instruction to reverse the order of the lanes, so in that case it might be necessary to manually reverse the lanes while loading using lsv:

lqv $v00, 0,s0         ; Load 8 16-bit samples from DMEM at address s0
lsv $v01,e(7),  0,s1   ; Load 8 16-bit samples from DMEM at address s1 in reverse order
lsv $v01,e(6),  2,s1
lsv $v01,e(5),  4,s1
lsv $v01,e(4),  6,s1
lsv $v01,e(3),  8,s1
lsv $v01,e(2), 10,s1
lsv $v01,e(1), 12,s1
lsv $v01,e(0), 14,s1

Scalar stores: SBV, SSV, SLV, SDV

31..26	25..21	20..16	15..11	10..7	6..0
SWC2	base	vt	opcode	element	offset

Assembly

ssv $v01,e(2), 0,s0    ; Store the 16-bit word in the third lane of $v01 into DMEM at address s0
sbv $v04,8, 0,s1       ; Store the 8-bit word in the 9th byte of $v04 (MSB of lane 4) into DMEM at address s1

Notice that it is possible to specify the lane syntax for the element field to refer to a specific lane, but if the access is made using slv or sdv (4 or 8 bytes), it will overflow into the following lanes.

Pseudo-code

addr = GPR[base] + offset * access_size
data = VPR[vt][element..element+access_size-1]
DMEM[addr..addr+access_size-1] = data

Description

These instructions store a scalar value (1, 2, 4, or 8 bytes) from a VPR into DMEM.

The address in DMEM where the value will be stored is computed as GPR[base] + (offset * access_size), where access_size is the number of bytes being accessed (eg: 4 for SLV). The address can be misaligned: despite how memory accesses usually work on MIPS, these instructions perform unaligned memory accesses.

The part of the vector register being accessed is VPR[vt][element..element+access_size], that is element selects the first accessed byte within the vector register. When element+access_size is bigger than 15, the element access wraps within the vector and a full-size store is always performed (eg: slv with element=15 stores VPR[vt][15,0..2] into memory, for a total of 4 bytes).

Usage

These instructions are seldom used. Normally, it is better to structure RSP code to work across full vectors to maximize parallelism. Data flow between RSP and VR4300 should be structured in vectorized format, so that it is possible to use a vector store (sqv, in case the output is made of 16-bit data) or a packed load (suv/spv, in case the output is made of 8-bit data). Consider also using mfc2 to store a 16-bit value from the lane of a VPR into a GPR.

See scalar loads for an example of a use-case (reversing a vector) that can be implemented also via ssv.

128-bit vector loads: LQV, LRV

31..26	25..21	20..16	15..11	10..7	6..0
LWC2	base	vt	opcode	element	offset

Insn	opcode	Desc
LQV	0x04	load (up to) 16 bytes into vector, left-aligned
LRV	0x05	load (up to) 16 bytes into vector, right-aligned

Assembly

// Standard 128-bit load from DMEM aligned address s0 into $v08
lqv $v08, 0,s0

// Loading a misaligned 128-bit vector from DMEM
// (a0 is 128-bit aligned in this example)
lqv $v00, 0x08,a0    // read bytes 0x08(a0) - 0x0F(a0) into left part of the vector (VPR[vt][0..7])
lrv $v00, 0x18,a0    // read bytes 0x10(a0) - 0x17(a0) into right part of the vector (VPR[vt][8..15])

// Advanced example using the "element" field
lqv $v08,e(2), 0x08,a0   // read bytes 0x08(a0) - 0x0F(a0) into VPR[vt][4..11]
lrv $v08,e(2), 0x18,a0   // read bytes 0x10(a0) - 0x13(a0) into VPR[vt][12..15]

Notice that the element field is optional (defaults to 0) and is usually not specified because these instructions are meant to affect the whole vector. The element field can be specified using the lane syntax (e(N)) or a raw number which maps to the byte offset inside the vector.

Pseudo-code

// lqv
addr = GPR[base] + (offset * 16)
end = addr | 15
size = MIN(end-addr, 15-element)
VPR[vt][element..element+size] = DMEM[addr..addr+size]

// lrv
end = GPR[base] + (offset * 16)
addr = end & ~16
size = MIN(end-addr, 15-element)
VPR[vt][element..addr+size] = DMEM[addr..addr+size]

Description

Roughly, these functions behave like lwl and lwr: combined, they allow to read 128 bits of data into a vector register, irrespective of the alignment.

When the data to be loaded is 128-bit aligned within DMEM, lqv is sufficient to read the whole vector (lrv in this case is redundant because it becomes a no-op).

The actual bytes accessed in DMEM depend on the instruction: for lwv, the bytes are those starting at GPR[base] + (offset * 16), up to and excluding the next 128-bit aligned byte (a0+0x10 in the above example); for lrv, the bytes are those starting at the previous 128-bit aligned byte (a0+0x10 in the above example) up to and excluding GPR[base] + (offset * 16). Again, this is exactly the same behavior of lwl and lwr, but for 128-bit aligned loads.

element is used as a byte offset within the vector register to specify the first byte affected by the operation; that is, the part of the vector being loaded with the instruction pair is VPR[vt][element..15]. Thus a non-zero element means that fewer bytes are loaded.

Usage

lqv is the most standard way to fill a full VPR vector register loading its contents from DMEM. Given that it's usually possible to define the layout of data in DMEM, it is advisable to design it so that vectors are always aligned to 128-bit (16 bytes), using the .align 4 directory: this allows to read the vector using just lqv, in 1 cycle (though the load has a 3-cycle latency like all instructions that write to a VPR).

    .data

    .align 4
CONST:  .half 3, 2, 7, 0, 0x4000, 0x8000, 0x7F, 0xFFF      # Several constants used for an algorithm

    .text
    
    lqv $v31, %lo(CONST),r0    # Load the constants

One example of using lqv and lrv in pair is to perform a fast memcpy from a possible misaligned address to an aligned destination buffer:

    # s0 is the pointer to source data in DMEM (possibly misaligned)
    # s4 will point to the destination buffer in DMEM (aligned)
    # t1 is the number of bytes to copy
    li s4, %lo(BUFFER)
loop:
    lqv $v00, 0x00,s0
    lrv $v00, 0x10,s0
    sqv $v00, 0,s4
    sub t1, 16
    add s0, 16
    bgtz t1, loop
    add s4, 16