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Copyright (C) 2020-2021 Intel Corporation

SPDX-License-Identifier: MIT

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## Opcode

  SCATTER4_SCALED = 0x75

## Format

| | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| 0x75(SCATTER4_SCALED) | Exec_size      | Pred | Channels | Scale | Surface | Offset |
|                       | Element_offset | Src  |          |       |         |        |


## 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 ch_start = ch_pos * max(exec_size, GRF_SIZE / 4);  // GRF_SIZE is the register size in bytes
              surface[(address/4)+c] = src[ch_start+i]; //4 bytes
            }
          }
          ch_pos++;
        }
      }

## Description


    Performs <exec_size>*<num_enabled_channels> element scattered write into <surface>, using the values from <src>. The address to be written 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. When scale is non-zero, only T0 (SLM) and T5 (stateless) are supported

- **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

- **Src(raw_operand):** The values to be written. For each enabled channel in RGBA order, exec_size elements will be written to the surface subject to predication. The next enabled channel will get its data from the next register. Must have type UD,D,F

#### Properties
- **Out-of-bound Access:** On write: data is dropped.


## Text
```
    

		[(<P>)] SCATTER4_SCALED.<channels> (<exec_size>) <surface> <offset> <element_offset> <src>
```



## Notes



                    The behavior is undefined if more than one channel writes to the same address.
