<!---======================= begin_copyright_notice ============================

Copyright (C) 2020-2022 Intel Corporation

SPDX-License-Identifier: MIT

============================= end_copyright_notice ==========================-->

## Opcode

  GATHER4_SCALED = 0x74

## Format

| | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 0x74(GATHER4_SCALED) | Exec_size      | Pred | Channels | Scale | Surface | Offset |
|                      | Element_offset | Dst  |          |       |         |        |


## Semantics


```

                    UD ch_pos = 0;
                    for (c = 0; c < 4; ++c) {
                      if (ch_mask[c] == 1) {
                        for (i = 0; i < exec_size; ++i) {
                          if (ChEn[i]) {
                            UD address = offset+element_offset[i]*(scale+1);
                            UD ch_start = ch_pos * max(exec_size, GRF_SIZE / 4);  // GRF_SIZE is the register size in byte
                            dst[ch_start+i] = surface[(address/4)+c]; //4 bytes
                          }
                        }
                        ch_pos++;
                      }
                    }
```

## Description





```
    Performs <exec_size>*<num_enabled_channels> element scattered read from <surface> and stores the results into <dst>. The address to be read is determined by the offset, scaling pitch, and each element offset, and the final address must be dword-aligned.
```


- **Exec_size(ub):** Execution size

  - Bit[2..0]: size of the region for source and destination operands

    - 0b011:  8 elements
    - 0b100:  16 elements
  - Bit[7..4]: execution mask (explicit control over the enabled channels)

    - 0b0000:  M1
    - 0b0001:  M2
    - 0b0010:  M3
    - 0b0011:  M4
    - 0b0100:  M5
    - 0b0101:  M6
    - 0b0110:  M7
    - 0b0111:  M8
    - 0b1000:  M1_NM
    - 0b1001:  M2_NM
    - 0b1010:  M3_NM
    - 0b1011:  M4_NM
    - 0b1100:  M5_NM
    - 0b1101:  M6_NM
    - 0b1110:  M7_NM
    - 0b1111:  M8_NM

- **Pred(uw):** Predication control


- **Channels(ub):**

  - Bit[3..0]: determines the write masks for the RGBA channel, with R being bit 0 and A bit 3. At least one channel must be enabled (i.e., "0000" is not allowed)


- **Scale(uw):** This field is ignored and scale is always zero


- **Surface(ub):** Index of the surface variable.


- **Offset(scalar):** The global byte offset. Must have type UD


- **Element_offset(raw_operand):** The first Exec_size elements of the operand will be used as the byte offsets into the surface. Must have type UD


- **Dst(raw_operand):** The variable storing the results of the read. For each enabled channel in RGBA order, exec_size elements will be returned to dst subject to predication. The next enabled channel will have its return data starting at the next register. If a channel's return data do not occupy the entire register, the remaining part of the register has undefined values. Must have type UD,D,F


#### Properties
- **Out-of-bound Access:** On read: zeros are returned.




## Text
```



    [(<P>)] GATHER4_SCALED.<channels> (<exec_size>) <surface> <offset> <element_offset> <dst>
```
## Notes





