Nested Instruction Loops in .NET framework

Create Denso QR Bar Code in .NET framework Nested Instruction Loops
Nested Instruction Loops
Scan Quick Response Code In Visual Studio .NET
Using Barcode Control SDK for VS .NET Control to generate, create, read, scan barcode image in VS .NET applications.
Once all the registers are set up correctly according to the assumptions made by STOSW, the real work of SHOWCHAR is performed by two instruction loops, one inside the other The inner loop displays a line consisting of 64 characters The outer loop breaks up the display into four such lines The inner loop is by far the more interesting of the two Here it is: DoChar: stosw jcxz AllDone inc AL dec BL loopnz DoChar ; ; ; ; ; Note that there's no REP prefix! When the full set is printed, quit Bump the character value in AL up by 1 Decrement the line counter by one Go back & do another char until BL goes to 0
Denso QR Bar Code Printer In .NET Framework
Using Barcode generator for .NET framework Control to generate, create QR Code image in .NET applications.
The work here (putting a character/attribute pair into the video buffer) is again done by STOSW Once again, STOSW is working solo, without REP Without REP to pull the loop inside the CPU, you have to set the loop up yourself Keep in mind what happens each time STOSW fires The character in AX is copied to ES:DI and DI is incremented by 2 At the other end of the loop, the LOOPNZ instruction decrements CX by 1 and closes the loop During register setup, we loaded CX with the number of characters we wanted to display-in this case, 256 Each time STOSW fires, it places another character on the screen, and there is one fewer character left to display CX acts as the master counter, keeping track of when we finally display the last remaining character When CX goes to zero, we've displayed the full ASCII character set and the job is done
QR Code Recognizer In Visual Studio .NET
Using Barcode reader for VS .NET Control to read, scan read, scan image in Visual Studio .NET applications.
Jumping When CX Goes to 0
Bar Code Drawer In Visual Studio .NET
Using Barcode encoder for .NET Control to generate, create barcode image in Visual Studio .NET applications.
Hence the instruction JCXZ This is a special branching instruction created specifically to help with loops like this Back in 10, I explained how it's possible to branch using one of the many variations of the JMP instruction, based on the state of one of the machine flags Earlier in this chapter, I explained the LOOP instruction, which is a special-purpose sort of a JMP instruction, one combined with an implied DEC CX instruction JCXZ is yet another variety of JMP instruction, but one that doesn't watch any of the flags or decrement any registers Instead, JCXZ watches the CX register When it sees that CX has just gone to zero, it jumps to the specified label If CX is still nonzero, execution falls through to the next instruction in line In the case of the inner loop shown previously, JCXZ branches to the "close up shop" code when it sees that CX has finally gone to 0 This is how the SHOWCHAR program terminates Most of the other JMP instructions have partners that branch when the governing flag is not true That is, JC (Jump on Carry) branches when the Carry flag equals 1 Its partner, JNC (Jump on Not Carry), jumps if the Carry flag is not 1 However, JCXZ is a loner There is no JCXNZ instruction, so don't go looking for one in the instruction reference!
Reading Barcode In .NET Framework
Using Barcode decoder for Visual Studio .NET Control to read, scan read, scan image in .NET applications.
Closing the Inner Loop
Print QR Code In C#
Using Barcode creator for .NET Control to generate, create QR-Code image in VS .NET applications.
Assuming that CX has not yet been decremented down to 0 by the STOSW instruction (a condition watched for by JCXZ), the loop continues AL is incremented This is how the next ASCII character in
Encoding QR Code ISO/IEC18004 In .NET
Using Barcode encoder for ASP.NET Control to generate, create Quick Response Code image in ASP.NET applications.
line is selected The value in AX is sent to the location at ES:DI by STOSW, and the character code proper is stored in AL If you increment the value in AL, you change the displayed character to the next one in line For example, if AL contains the value for the character A (65), incrementing AL changes the A character to a B (66) On the next pass through the loop, STOSW will fire a B at the screen instead of an A Why not just increment AX The AH half of AX contains the attribute byte, and we do not want to change that By explicitly incrementing AL instead of AX, we ensure that AH will never be altered After the character code in AL is incremented, BL is decremented Now, BL is not directly related to the string instructions Nothing in any of the assumptions made by the string instructions involves BL We're using BL for something else entirely here BL is acting as a counter that governs the length of the lines of characters shown on the screen BL was loaded earlier with the value represented by LineLen; here, 64 On each pass through the loop, the DEC BL instruction decrements the value of BL by 1 Then the LOOPNZ instruction gets its moment in the sun LOOPNZ is a little bit different from our friend LOOP that we examined earlier It's just different enough to get you into trouble if you don't truly understand how it works Both LOOP and LOOPNZ decrement the CX register by 1 LOOP watches the state of the CX register and closes the loop until CX goes to 0 LOOPNZ watches both the state of the CX register and the state of the Zero flag ZF (LOOP ignores ZF) LOOPNZ will only close the loop if CX <> 0 and ZF = 0 In other words, LOOPNZ closes the loop only if CX still has something left in it, and if the Zero flag ZF is not set So, what exactly is LOOPNZ watching for here Remember that immediately prior to the LOOPNZ instruction, we're decrementing BL by 1 through a DEC BL instruction The DEC instruction always affects ZF If DEC's operand goes to zero as a result of the DEC instruction, ZF goes to 1 (is set) Otherwise, ZF stays at 0 (remains cleared) So, in effect, LOOPNZ is watching the state of the BL register Until BL is decremented to 0 (setting ZF), LOOPNZ closes the loop After BL goes to zero, the inner loop is finished and execution falls through LOOPNZ to the next instruction What about CX Well, LOOPNZ is watching CX-but so is JCXZ JCXZ is actually the switch that governs when the whole loop-both inner and outer portions-has done its work and must stop So, while LOOPNZ does watch CX, somebody else is doing that task, and that somebody else will take action on CX before LOOPNZ can LOOPNZ's job is thus to decrement CX, but to watch BL It governs the inner of the two loops
Encoding QR-Code In Visual Basic .NET
Using Barcode printer for Visual Studio .NET Control to generate, create Denso QR Bar Code image in .NET framework applications.
UCC - 12 Creator In Visual Studio .NET
Using Barcode maker for .NET framework Control to generate, create UPC Symbol image in .NET framework applications.
Creating Barcode In Visual Studio .NET
Using Barcode encoder for Visual Studio .NET Control to generate, create bar code image in .NET framework applications.
Printing Barcode In Java
Using Barcode generator for Java Control to generate, create bar code image in Java applications.
Generate Code 128 Code Set B In VB.NET
Using Barcode drawer for Visual Studio .NET Control to generate, create Code 128C image in Visual Studio .NET applications.
Generating Barcode In C#.NET
Using Barcode creation for Visual Studio .NET Control to generate, create barcode image in Visual Studio .NET applications.
Bar Code Creator In VS .NET
Using Barcode drawer for ASP.NET Control to generate, create barcode image in ASP.NET applications.