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tevador
2019-05-03 22:09:52 +02:00
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doc/specs.md
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@@ -622,20 +622,20 @@ Whenever a register is selected as the operand of a CBRANCH instruction, its `co
The CBRANCH instruction performs the following steps:
1. A constant `b` is calculated as `mod.cond + RANDOMX_JUMP_OFFSET`.
1. A constant `conditionImmediate` is constructed as sign-extended `imm32` with bit `b` set to 1 and bit `b-1` set to 0 (if `b > 0`).
1. `conditionImmediate` is added to `creg`.
1. A constant `cimm` is constructed as sign-extended `imm32` with bit `b` set to 1 and bit `b-1` set to 0 (if `b > 0`).
1. `cimm` is added to `creg`.
1. If bits `b` to `b + RANDOMX_JUMP_BITS - 1` of `creg` are zero, execution jumps to instruction `creg.lastUsed + 1` (the instruction following the instruction where `creg` was last modified).
Bits in immediate and register values are numbered from 0 to 63 with 0 being the least significant bit. For example, for `b = 10` and `RANDOMX_JUMP_BITS = 8`, the bits are arranged like this:
```
conditionImmediate = SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSMMMMMMMMMMMMMMMMMMMMM10MMMMMMMMM
creg = ..............................................XXXXXXXX..........
cimm = SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSMMMMMMMMMMMMMMMMMMMMM10MMMMMMMMM
creg = ..............................................XXXXXXXX..........
```
`S` is a copied sign bit from `imm32`. `M` denotes bits of `imm32`. The 9th bit is set to 0 and the 10th bit is set to 1. This value would be added to `creg`.
The second line uses `X` to mark bits of `creg` that would be checked by the condition. If all these bits are 0 after adding `conditionImmediate`, the jump is executed.
The second line uses `X` to mark bits of `creg` that would be checked by the condition. If all these bits are 0 after adding `cimm`, the jump is executed.
The construction of the CBRANCH instruction ensures that no inifinite loops are possible in the program.