dcbst (Data Cache 块存储器) 指令

用途

允许程序将修改后的块的内容复制到主存储器。

注: 仅在 PowerPC® 体系结构中支持 dcbst 指令。

语法

描述

dcbst 指令导致将块的任何修改副本复制到主内存。 如果 RA 不是 0 ，那么 dcbst 指令通过将通用寄存器 (GPR) RA 的内容添加到 GPR RB的内容来计算有效地址 (EA)。 否则， EA 是 RB的内容。 如果包含寻址字节的高速缓存块位于数据高速缓存中并已修改，那么会将该块复制到主内存。

可以使用 dcbst 指令来确保主内存中某个位置的副本包含最新的更新。 当与不参与一致性协议的 I/O 设备共享内存时，这可能很重要。 此外， dcbst 指令可以确保将更新立即复制到图形帧缓冲区。

在地址转换和保护方面，将 dcbst 指令视为来自寻址字节的负载。

dcbst 指令有一种语法形式，不会影响定点异常寄存器。

参数

示例

1. 以下代码与未参与一致性协议的 I/O 设备共享内存:

   
   # Assume that location A is memory that is shared with the
   # I/O device.
   # Assume that GPR 2 contains a control value indicating that
   # and I/O operation should start.
   # Assume that GPR 3 contains the new value to be placed in
   # location A.
   # Assume that GPR 4 contains the address of location A.
   # Assume that GPR 5 contains the address of a control register
   # in the I/O device.
   st      3,0,4           # Update location A.
   dcbst   0,4             # Copy new content of location A and
                           # other bytes in cache block to main
                           # memory.
   sync                    # Ensure the dcbst instruction has 
                           # completed.
   st      2,0,5           # Signal I/O device that location A has
                           # been update.
   

2. 以下代码将复制到图形帧缓冲区，确保不延迟地显示新值:

   
   # Assume that target memory is a graphics frame buffer.
   # Assume that GPR 2, 3, and 4 contain new values to be displayed.
   # Assume that GPR 5 contains the address minus 4 of where the
   # first value is to be stored.
   # Assume that the 3 target locations are known to be in a single
   # cache block.
   addi    6,5,4           # Compute address of first memory
                           # location.
   stwu    2,4(5)          # Store value and update address ptr.
   stwu    3,4(5)          # Store value and update address ptr.
   stwu    4,4(5)          # Store value and update address ptr.
   dcbst   0,6             # Copy new content of cache block to
                           # frame buffer. New values are displayed.