Jcxz instructional design

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JCXZ/JECXZ - Jump if Register (E)CX is Zero Usage: JCXZ label JECXZ label (386+) Modifies flags: None Causes execution to branch to label if register CX is zero. Uses unsigned comparision. Clocks Size Operands 808x 286 386 486 Bytes label: jump 18 8+m 9+m 8 2 no jump 6 4 5 5 JCXZ (Jump if ECX Zero) branches to the label specified in the instruction if it finds a value of zero in ECX. JCXZ is useful in combination with the LOOP instruction and with the string scan and compare instructions, all of which decrement ECX. Sometimes, it is desirable to design a loop that executes zero times if the count Notes: Many instructions use CX as a count register, and JCXZ allows you to test and jump to see if CX has become 0. The jump may only be a short jump (that is, no more than 127 bytes forward or 128 bytes back) and will be taken if CX = 0 at the time the instruction is executed. If CX is any other value than 0, execution Jump if ECX is Zero (jcxz). jcxz disp8. Operation. jump to disp8 if (E)CX is 0. Description. The jcxz instruction tests the contents of the CX or ECX register for 0. jcxz differs from other conditional jumps that it tests the flags, rather than (E)CX. jcxz is useful at the beginning of a loop that terminates with a conditional loop 5 Feb 2010 The loop instruction automatically decrements cx, and only jumps if cx != 0. There are also LOOPE, and LOOPNE variants, if you want to do some additional check for your loop to break out early. If you want to modify cx during your loop, make sure to push it onto the stack before the loop content, and pop it 13 Mar 2014 Remember that je is really just a synonym of jz , it doesn't really care about equality of any form, it just looks at the Z flag. 1234h minus itself is obviously zero, so the Z flag should be set. In the second example, jcxz does branch if cx = 0 , which is the case. After incrementing cx , it will be zero, because it was CHAPTER SIX: THE 80x86 INSTRUCTION SET (Part 5). 6.9 - Program Flow Control Instructions 6.9.1 - Unconditional Jumps 6.9.2 - The CALL and RET Instructions 6.9.3 - The INT INTO BOUND and IRET Instructions 6.9.4 - The Conditional Jump Instructions, 6.9.5 - The JCXZ/JECXZ Instructions 6.9.6 - The LOOP Instruction 18 Apr 2015 The revised edition closely integrates instruction and practice, learning and application. Creative innovations have been incorporated in the course content, general arrangements, page design, question format and instructional concepts. The following is a summary of major changes including reorganization, deletion and. D. A. Patterson and J. L. Hennessy, Computer Organization and Design: The Hardware/ Software. Interface, 2nd ed., San 2.2 Repeat Exercise 2.1 for the JCXZ instruction in Table 2.3. (§2.1). 2.3 According 2.10 Suppose the instruction word in a general register machine has space for an opcode and either three register