深入理解计算机系统chapter4.ppt

A similar ambiguity occurs for the instruction popl %esp. It could either set %esp to the value read from memory or to the incremented stack pointer. 1 .text 2 .globl pushtest 3 pushtest: 4 pushl %ebp 5 movl %esp, %ebp 6 movl %esp, %eax Copy stack pointer 7 pushl %esp Push stack pointer 8 popl %edx Pop it back 9 subl %edx,%eax Subtract new from old stack pointer 10 leave Restore stack frame pointers 11 ret In our experiments, we find that function pushtest always returns zero. Practice Problem 4.6 4.1.6 Some Y86 Instruction Details Practice Problem 4.7 1 .text 2 .globl poptest 3 poptest: 4 pushl %ebp 5 movl %esp, %ebp 6 pushl $0xabcd Push test value 7 popl %esp Pop to stack pointer 8 movl %esp, %eax Set popped value as return value 9 leave Restore stack and frame pointers 10 ret In our experiments, We find this function always returns 0xabcd. 4.1.6 Some Y86 Instruction Details For IA-32 processors from the Intel 286 on, the PUSH ESP instruction pushes the value of the ESP register as it existed before the instruction was executed. For the IntelR 8086 processor, the PUSH SP instruction pushes the new value of the SP register. What this note states is that different models of x86 processors do different things when instructed to push the stack pointer register. Some push the original value, while others push the decremented value. There are two drawbacks to this inconsistency （不一致） : It decreases code portability. It complicates （使复杂） the documentation. Aside Getting the details right: Inconsistencies across x86 models 4.1.6 Some Y86 Instruction Details 4.2 Logic Design and the Hardware Control Language HCL In hardware design, electronic circuits are used to compute functions on bits and to store bits in different kinds of memory elements. Most contemporary circuit technology represents different bit values as high or low voltages （电压 ）on signal wires. In current technology, logic value 1 is represented by a high voltage of around 1.0 volt, while log