首页 随笔 乐走天涯 程序资料 评论中心 Tag 论坛 其他资源 搜索 消息中心 联系我 关于 RSS

9. Address generation interlock (PPlain and PMMX)


日期: 2000-04-01 14:00 | 联系我 | 关注我: SteemIT, Twitter, Google+

9. Address generation interlock (PPlain and PMMX)

It takes one clock cycle to calculate the address needed by an instruction which accesses memor y. Normally, this calculation is done at a separate stage in the pipeline while the preceding instruction or instruction pair is executing. But if the address depends on the result of an instruction executing in the preceding clock cycle, then you have to wait an extra clock cycle for the address to be calculated. This is called an AGI stall. Example:

ADD EBX,4 / MOV EAX,[EBX] ; AGI stall

The stall in this example can be removed by putting some other instructions in between ADD EBX,4 and MOV EAX,[EBX] or by rewriting the code to: MOV EAX,[EBX+4] / ADD EBX,4

You can also get an AGI stall with instructions which use ESP implicitly for addressing, such as PUSH, POP, CALL, and RET, if ESP has been changed in the preceding clock cycle by instructions such as MOV, ADD, or SUB. The PPlain and PMMX have special circuitry to predict the value of ESP after a stack operation so that you do not get an AGI delay after changing ESP with PUSH, POP, or CALL. You can get an AGI stall after RET only if it has an immediate operand to add to ESP.

Examples:

ADD ESP,4 / POP ESI ; AGI stall POP EAX / POP ESI ; no stall, pair MOV ESP,EBP / RET ; AGI stall CALL L1 / L1: MOV EAX,[ESP+8] ; no stall RET / POP EAX ; no stall RET 8 / POP EAX ; AGI stall

The LEA instruction is also subject to an AGI stall if it uses a base or index register which has been changed in the preceding clock cycle. Example:

INC ESI / LEA EAX,[EBX+4*ESI] ; AGI stall

PPro, PII and PIII have no AGI stalls for memory reads and LEA, but they do have AGI stalls for memory writes. This is not very significant unless the subsequent code has to wait for the write to finish.

标签: MMX 优化

 文章评论
目前没有任何评论.

↓ 快抢占第1楼,发表你的评论和意见 ↓

发表你的评论
如果你想针对此文发表评论, 请填写下列表单:
姓名: * 必填 (Twitter 用户可输入以 @ 开头的用户名, Steemit 用户可输入 @@ 开头的用户名)
E-mail: 可选 (不会被公开。如果我回复了你的评论,你将会收到邮件通知)
网站 / Blog: 可选
反垃圾广告: 为了防止广告机器人自动发贴, 请计算下列表达式的值:
1 x 4 + 1 = * 必填
评论内容:
* 必填
你可以使用下列标签修饰文字:
[b] 文字 [/b]: 加粗文字
[quote] 文字 [/quote]: 引用文字

 
首页 随笔 乐走天涯 猎户星 Google Earth 程序资料 程序生活 评论 Tag 论坛 资源 搜索 联系 关于 隐私声明 版权声明 订阅邮件

程序员小辉 建站于 1997 ◇ 做一名最好的开发者是我不变的理想。
Copyright © XiaoHui.com; 保留所有权利。