start adding demo of backward-compatible compressed encoding

diff --git a/openpower/sv/16_bit_compressed.mdwn b/openpower/sv/16_bit_compressed.mdwn
index 943d9de5bed028e8ec0d031f7f136863958676c9..f9f019763cc5359c712c6802c5ead0b5112f0d20 100644 (file)
--- a/openpower/sv/16_bit_compressed.mdwn
+++ b/openpower/sv/16_bit_compressed.mdwn
@@ -628,3 +628,120 @@ the register allocation order in GCC (i.e., it takes the earliest available regi
        vrsave, vscr    (fixed)
        sfp             (fixed)
 
+# Demo of encoding that's backward-compatible with PowerISA v3.1 in both LE and BE mode
+
+## Assumed instruction encodings for this encoding demo
+
+Bits numbered using MSB0. Ranges are inclusive.
+
+### Small Register Field Encoding
+
+| Encoding | Register |
+|----------|----------|
+| 0        | R3       |
+| 1        | R4       |
+| 2        | R5       |
+| 3        | R6       |
+| 4        | R28      |
+| 5        | R29      |
+| 6        | R30      |
+| 7        | R31      |
+
+### Instruction Fields
+
+| Name | Range        | Standard      | Description                                         |
+|------|--------------|---------------|-----------------------------------------------------|
+| PO   | 0:5          | PowerISA v3.0 | Primary Opcode                                      |
+| XO   | 21:30        | PowerISA v3.0 | Extended Opcode                                     |
+| SCM  | 15           | New           | Swap between Compressed & standard Mode             |
+| SCMT | 0            | New           | Swap between Compressed & standard Mode Temporarily |
+| SI   | 16:31        | PowerISA v3.0 | Signed Immediate                                    |
+| si0  | prefix 14:31 | PowerISA v3.1 | upper 18 bits Signed Immediate                      |
+| si1  | suffix 16:31 | PowerISA v3.1 | lower 16 bits Signed Immediate                      |
+| SO   | 6:8          | New           | Small Opcode for 16-bit instructions                |
+| SOH  | 1:5          | New           | Small Opcode High bits for 16-bit instructions      |
+| SRS  | 12:14        | New           | Small Source Register for 16-bit instructions       |
+| SRT  | 9:11         | New           | Small Target Register for 16-bit instructions       |
+
+### Mnemonic Prefixes
+
+| Prefix     | Mode       | Next Mode  | Next Next Mode | Instruction Width | Example                 | Comment                                        |
+|------------|------------|------------|----------------|-------------------|-------------------------|------------------------------------------------|
+| _\<None\>_ | Standard   | Standard   | -              | 32-bit            | `addi R3, R3, 0x15`     | Defined by PowerISA v3.0                       |
+| `p`        | Standard   | Standard   | -              | 64-bit            | `paddi R3, R3, 0x12345` | Defined by PowerISA v3.1                       |
+| `x.`       | Standard   | Standard   | -              | 48-bit            | `x.placeholder`         | `x` means heXa-byte                            |
+| `h.`       | Standard   | Standard   | -              | 16-bit            | `h.add R3, R4`          | `h` means Half-word                            |
+| `hs.`      | Standard   | Compressed | -              | 16-bit            | `hs.add R3, R4`         | `hs` means Half-word & Swap mode               |
+| `c.`       | Compressed | Compressed | -              | 16-bit            | `c.addi R3, R4, 0x5`    | `c` means Compressed                           |
+| `cs.`      | Compressed | Standard   | -              | 16-bit            | `cs.add R3, R4`         | `cs` means Compressed & Swap mode              |
+| `cst.`     | Compressed | Standard   | Compressed     | 16-bit            | `cst.add R3, R4`        | `cst` means Compressed & Swap mode Temporarily |
+
+### Standard Mode
+
+#### Standard Mode 32-bit Instructions (PowerISA v3.0)
+
+| Mnemonic             | PO (0:5) | RT (6:10) | RA (11:15) | SI (16:31) | Operation                      |
+|----------------------|----------|-----------|------------|------------|--------------------------------|
+| `addi RT, RA, immed` | 14       | RT        | RA         | SI         | RT <- (RA&#x7C;0) + EXTS64(SI) |
+
+#### Standard Mode 64-bit Instructions (PowerISA v3.1)
+
+Note that bit numbering goes from 0 to 31 in the prefix, then 0 to 31 in the suffix -- following the numbering in the PowerISA v3.1 spec.
+
+| Field       | Value                                          |
+|-------------|------------------------------------------------|
+| Mnemonic    | `paddi RT, RA, immed`                          |
+| PO (0:5)    | 14                                             |
+| (6:7)       | 2                                              |
+| (8)         | 0                                              |
+| (9:10)      | //                                             |
+| R (11)      | 0                                              |
+| (12:13)     | //                                             |
+| si0 (14:31) | si0                                            |
+| PO (0:5)    | 14                                             |
+| RT (6:10)   | RT                                             |
+| RA (11:15)  | RA                                             |
+| si1 (16:31) | si1                                            |
+| Operation   | RT <- (RA&#x7C;0) + EXTS64(si0&#x7C;&#x7C;si1) |
+
+#### Standard Mode 16-bit Instructions (New)
+
+| Mnemonic      | PO (0:5) | SO (6:8) | SRT (9:11) | SRS (12:14) | SCM (15) | Operation                                   |
+|---------------|----------|----------|------------|-------------|----------|---------------------------------------------|
+| h.add RT, RS  | 5        | 0        | RT         | RS          | 0        | RT <- RT + RS                               |
+| hs.add RT, RS | 5        | 0        | RT         | RS          | 1        | RT <- RT + RS <br/> Swap to Compressed Mode |
+
+#### Standard Mode 48-bit Instructions (New; Placeholder)
+
+| Mnemonic      | PO (0:5) | placeholder (6:20) | XO (21:30) | placeholder (31:47) | Operation   |
+|---------------|----------|--------------------|------------|---------------------|-------------|
+| x.placeholder | 0        | placeholder        | != 256     | placeholder         | placeholder |
+
+### Compressed Mode
+
+#### Compressed Mode 16-bit Instructions (New)
+
+| Mnemonic       | SCMT (0) | SOH (1:5) | SO (6:8) | SRT (9:11) | SRS (12:14) | SCM (15) | Operation                                               |
+|----------------|----------|-----------|----------|------------|-------------|----------|---------------------------------------------------------|
+| c.add RT, RS   | 0        | 1         | 0        | RT         | RS          | 0        | RT <- RT + RS                                           |
+| cs.add RT, RS  | 0        | 1         | 0        | RT         | RS          | 1        | RT <- RT + RS <br/> Swap to Compressed Mode             |
+| cst.add RT, RS | 1        | 1         | 0        | RT         | RS          | 1        | RT <- RT + RS <br/> Swap to Compressed Mode Temporarily |
+
+## LE mode
+
+TODO(programmerjake): finish
+
+| Address | Byte | Description |
+|---------|------|-------------|
+| 0x1000  |      |             |
+| 0x1001  |      |             |
+
+## BE mode
+
+TODO(programmerjake): finish
+
+| Address | Byte | Description |
+|---------|------|-------------|
+| 0x1000  |      |             |
+| 0x1001  |      |             |
+