mbar - 2024.2 English

Decsription

This instruction ensures that outstanding memory accesses on memory interfaces are completed before any subsequent instructions are executed. This is necessary to guarantee that self-modifying code is handled correctly, and that a DMA transfer can be safely started.

With self-modifying code, it is necessary to first use an MBAR instruction to wait for data accesses, which can be done by setting IMM to 1, and then use another MBAR instruction to clear the Branch Target Cache and empty the instruction prefetch buffer, which can be done by setting IMM to 2.

To ensure that data to be read by a DMA unit has been written to memory, it is only necessary to wait for data accesses, which can be done by setting IMM to 1.

When MicroBlaze is configured to use an MMU (C_USE_MMU >= 1) this instruction is privileged when the most significant bit in IMM is set to 1. This means that if the instruction is attempted in User Mode (MSR[UM] = 1) a Privileged Instruction exception occurs.

When the two most significant bits in IMM are set to 10 (Sleep), 01 (Hibernate), or 11 (Suspend) and no exception occurs, MicroBlaze enters sleep mode after all outstanding accesses are complete, and sets the Sleep, Hibernate, or Suspend output signal respectively to indicate this. The pipeline is halted, and MicroBlaze will not continue execution until a bit in the Wakeup input signal is asserted.

Pseudocode

if (IMM & 1) = 0 then
   wait for instruction side memory accesses
if (IMM & 2) = 0 then
   wait for data side memory accesses
PC   ←   PC + 4
if (IMM & 24)≠ 0 then
   enter sleep mode

Registers Altered

• PC
• ESR[EC], in case a privileged instruction exception is generated

Latency

• 2 + N cycles when C_INTERCONNECT = 2 (AXI)
• 8 + N cycles when C_INTERCONNECT = 3 (ACE)

N is the number of cycles to wait for memory accesses to complete.

Notes

• This instruction must not be preceded by an imm instruction, and must not be placed in a delay slot.
• The assembler pseudo-instructions sleep, hibernate, and suspend can be used instead of “mbar 16”, “mbar 8”, and “mbar 24” respectively to enter sleep mode.