Bungie Day!

Bungie celebrated Bungie Day last week (07/07/2007 - 777), weird but it may well be the day when most get married (http://www.time.com/time/business/article/0,8599,1630320,00.html). They released a new theme and gamerpics on that day, and that day only. Listed below are the blades.

Modifing the size of your integer value

When moving a integer, from say a smaller size field (word) to say a doubleword, you should not just movw to the new register.

movw %ax, %bx

This should not be done because you can not be certain that the upper part of the EBX register is zeroed out ahead of time. To do this you should first zero out the EBX (destination) and then move your intended value there.

movl $0, %ebx
movw %ax, %bx

Intel provides another instruction that can do this with one instruction, movzx. It takes a source (a register or memory location) and converts to a larger size (register only) destination.

movl $300, %ecx
movzx %cl, %ebx

Just thought this was interesting.

Under the C++ covers....literally

In my continuing studies of C++ I have been studing assembly language. Not from a standpoint that I will be authoring large programs written totally in assembly, but when you are down to your last options and trying to squeeze performance from a piece of code, taking a look at the assembly generated by the HLL (high level language) you can find things otherwise unseen.

Studies on Assembly: (get the cpu vendor id info)

#cpuid - program to get the processor vendor id .section .data output: .ascii "Processor Vendor ID is 'xxxxxxxxxxxx'\n" .section .text .globl _start _start: movl $0, %eax cpuid movl $output, %edi movl %ebx, 28(%edi) movl %edx, 32(%edi) movl %ecx, 36(%edi) movl $4, %eax movl $1, %ebx movl $output, %ecx movl $42, %edx int $0x80 movl $1, %eax movl $0, %ebx int $0x80

This is a pretty simple assembly program. In the .data section there is a output label (think variable) defined. The .ascii is the "type" of data. In this case it means store ASCII string. The x's are placeholders for the real value coming later. The space for this data is reserved at compile time. The next section, .text, is where the instructions are stored for the program. The first instructions are to load the register EAX with a value of zero (literal). The next instruction is cpuid, which instructs the processor to get the id we are after. The zero value in EAX defines the CPUID output option. After the CPUID instruction is run, we must collect the result which will be in 3 output registers. The first instruction here (movl $output, $edi) creates a pointer from the output label to a register (EDI). Next we create pointers from the other 3 registers to the appropriate section of the EDI register. Now that all the results have been coorelated we can output the response. This program is using a Linux system call (int $0x80) to access the console from the kernel. This is a software interrupt (with the value 0x80). The EAX register will hold the specific instruction that will be executed when we make this interrupt. The EBX will hold the exit code that is given when the program exits. The ECX will hold the actual output (movl $output, %ecx). And the EDX register will hold the length of the string. Thats pretty much it. I will be posting more as my study of C++ and assembly continues.

Torque X for XNA creators (FREE!)

Check out one of the coolest benefits of becoming a XNA Creator Club member (http://creators.xna.com/subscribers/torquex.aspx). This is makes the $100 entry to creators club easier to swallow for some. Content is being added to the site daily (new tutorials and info). Hats off to the XNA Dev Team at Microsoft!