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Minor README changes

This commit is contained in:
Clifford Wolf 2015-06-28 01:18:19 +02:00
parent df6b95fb87
commit 792baeabcf
1 changed files with 13 additions and 12 deletions
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README.md
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@ -104,7 +104,7 @@ Set this to 1 to enable the Pico Co-Processor Interface (PCPI).
This parameter is only available on the `picorv32_axi` core. It internally
enables PCPI and instantiates the `picorv32_pcpi_mul` core that implements
Mthe `MUL[H[SU|U]]` instructions. The external CPCI interface only becomes
the `MUL[H[SU|U]]` instructions. The external CPCI interface only becomes
functional when ENABLE_PCPI is set as well.
#### ENABLE_IRQ (default = 0)
@ -198,18 +198,20 @@ This interrupts can also be triggered by external sources, such as co-processors
connected via PCPI.
The core has 4 additional 32-bit registers `q0 .. q3` that are used for IRQ
handling. When the IRQ handler is called, the register `q0` contains the return address
and `q1` contains a bitmask of all IRQs to be handled. This means one call to the interrupt
handler might need to service more than one IRQ when more than one bit is set
in `q1`.
handling. When the IRQ handler is called, the register `q0` contains the return
address and `q1` contains a bitmask of all IRQs to be handled. This means one
call to the interrupt handler needs to service more than one IRQ when more than
one bit is set in `q1`.
Registers `q2` and `q3` are uninitialized and can be used as temporary storage
when saving/restoring register values in the IRQ handler.
All of the following instructions are encoded under the `custom0` opcode.
#### getq rd, qs
This instruction copies the value from a q-register to a general-purpose
register. This instruction is encoded under the `custom0` opcode:
register.
0000000 00000 000XX 000 XXXXX 0001011
f7 rs2 qs f3 rd opcode
@ -225,7 +227,7 @@ Example assembler code using the `custom0` mnemonic:
#### setq qd, rs
This instruction copies the value from a general-purpose register to a
q-register. This instruction is encoded under the `custom0` opcode:
q-register.
0000001 00000 XXXXX 000 000XX 0001011
f7 rs2 rs f3 qd opcode
@ -241,8 +243,7 @@ Example assembler code using the `custom0` mnemonic:
#### retirq
Return from interrupt. This instruction copies the value from `q0`
to the program counter and re-enables interrupts. This instruction is
encoded under the `custom0` opcode:
to the program counter and re-enables interrupts.
0000010 00000 00000 000 00000 0001011
f7 rs2 rs f3 rd opcode
@ -257,7 +258,7 @@ Example assembler code using the `custom0` mnemonic:
The "IRQ Mask" register contains a bitmask of masked (disabled) interrupts.
This instruction writes a new value to the irq mask register and reads the old
value. This instruction is encoded under the `custom0` opcode:
value.
0000011 00000 XXXXX 000 XXXXX 0001011
f7 rs2 rs f3 rd opcode
@ -275,8 +276,8 @@ interrupt is disabled will cause the processor to halt.
#### waitirq
Pause execution until an interrupt triggers. This instruction is encoded under the
`custom0` opcode. The bitmask of pending IRQs is written to `rd`.
Pause execution until an interrupt triggers. The bitmask of pending IRQs is
written to `rd`.
0000100 00000 00000 000 XXXXX 0001011
f7 rs2 rs f3 rd opcode