问题
单项选择题
在减法指令中，______指令不会影响CF标志位。
A．SUB
B．SBB
C．DEC
D．NEG
答案
参考答案：C
解析： DEC指令是将操作数减1，它不会影响CF，SBB是带借位的减法指令，NEG其实就是取补指令，和SUB一样，它们都会影响CF标志位。
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在减法指令中，______指令不会影响CF标志位。
A．SUB
B．SBB
C．DEC
D．NEG
参考答案：C
解析： DEC指令是将操作数减1，它不会影响CF，SBB是带借位的减法指令，NEG其实就是取补指令，和SUB一样，它们都会影响CF标志位。
Passage Three
| A rapid means of long-distance transportation became a necessity for the United States as settlement (新拓居地) spread ever farther westward. The early trains were impractical curiosities, and for a long time the railroad companies met with troublesome mechanical problems. The most serious ones were the construction of rails able to bear the load, and the development of a safe, effective stopping system. Once these were solved, the railroad was established as the best means of land transportation. By 1860 there were thousands of miles of railroads crossing the eastern mountain ranges and reaching westward to the Mississippi. There were also regional southern and western lines. The high point in railroad building came with the construction of the first transcontinental system. In 1862 Congress authorized two western railroad companies to build lines from Nebraska westward and from California eastward to a meeting point, so as to complete a transcontinental crossing linking the Atlantic seaboard with the Pacific. The Government helped the railroads generously with money and land. Actual work on this project began four years later. The Central Pacific Company, starting from California, used Chinese labor, while the Union Pacific employed crews of Irish laborers. The two groups worked at remarkable speed, each trying to cover a greater distance than the other. In 1869 they met at a place called Promontory in what is now the state of Utah. Many visitors came there for the great occasion. There were joys all over the country, with parades and the ringing of church bells to honor the great achievement. The railroad was very important in encouraging westward movement. It also helped build up industry and farming by moving raw materials and by distributing products rapidly to distant markets. In linking towns and people to one another it helped unify the United States. |
The building of the first transcontinental system ______.
A．relieved the burden of overall railroad system
B．encouraged people to rush for gold in the west
C．attracted many visitors to the construction sites
D．brought about a rapid growth of industry and farming in the west