Bug 757

Summary: bitmanip instruction encodings -- adjust to match existing X/XO-forms?
Product: Libre-SOC's second ASIC Reporter: Jacob Lifshay <programmerjake>
Component: source codeAssignee: Luke Kenneth Casson Leighton <lkcl>
Status: RESOLVED FIXED    
Severity: enhancement CC: libre-soc-bugs, lkcl, programmerjake
Priority: ---    
Version: unspecified   
Hardware: PC   
OS: Linux   
NLnet milestone: --- total budget (EUR) for completion of task and all subtasks: 0
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Bug Depends on:    
Bug Blocks: 741, 755    

Description Jacob Lifshay 2021-12-17 03:56:23 GMT 
Currently, the encoding for bitmanip instructions such as grevi have the sh fields split across several different ranges, bits 16 to 20 and bit 23. Also the XO field is split across 21 to 22 and 24 to 30. This has little in common with any existing instructions.

0.5  6.10  11.15  16.20  21.22  23  24....30  31  name
NN   RA    RB     sh     01     SH  1010 110  Rc  grevi

I propose that we instead use an encoding like the XO and X forms:
# 1.6.16 XO-FORM (trimmed)
   |0     |6   |11   |16     |21 |22    |31  |
   | PO   |  RT|   RA|   RB  |OE |   XO |Rc  |

# 1.6.7 X-FORM (trimmed)
   |0     |6   |11  |16  |21   |31  |
   | PO   | RT | RA | RB |  XO |Rc  |


Proposed encodings:
   |0     |6   |11   |16     |21 |22    |31  |
   | PO   |  RT|   RA|   SH      |   XO |Rc  | grevi (kinda XO-form)
   | PO   |  RT|   RA|   RB  |       XO |Rc  | grev (X-form)

To get the new encoding, take the old encoding and remove bit 23, inserting it before bit 21, so it becomes the new bit 21 and bit 21:22 are shifted over 1 to 22:23.
Comment 1 Jacob Lifshay 2022-01-06 03:00:29 GMT 
Well, since no one apparently has any comments, I'm going to just make the changes I proposed.