The addq a,b instruction adds the contents of a to b. So if rcx = 0x1, rax = 0x100 and we have 0xff at address 0x100, then add %rcx,(%rax) adds 0x1 in rcx to 0xff at address 0x100, yielding 0x100 at address 0x100 as the worksheet correctly indicates. Your intuition would be correct if the instruction was movq instead of addq as movq just overwrites a memory location or register instead of adding to it.
Use cdq / idiv for signed 32-bit / 32-bit => 32 bit division,xor edx,edx / div for unsigned. With the dividend in EAX to start with, and the divisor specified as an operand to DIV or IDIV. If you zero EDX/RDX instead of sign-extending into EDX:EAX before idiv, you can get a large positive result for -5 / 2, for example. Using the “full … Read more
I’m assuming that the pseudocode executes sequentially, so an earlier condition being true means you go there and never reach the later if statements. This makes the last branch guaranteed taken if it’s reached at all, so it doesn’t even need to be conditional. (Also assuming that this is a MIPS without a branch-delay slot.) … Read more
I keep seeing this form often: div 3, 15 this is not any valid INTEL mneumonic! To divide 15 by 3: For the second error, you cannot move a 16 bit register into a 32 bit register like that. You need to use one of the following: Or:
For the 1st error you forgot to type a comma between the register and the immediate. For the 2nd and 3rd errors the CH and CL registers cannot be used for addressing memory. Use SI, DI, or BX instead. Since your array is defined as words you must treat it as such!Change into something like … Read more
4097 = 1001 hex so, the first instruction puts 0x10010000 into register t0. lui is “load upper immediate”, with “upper” meaning the upper 16 bits, and “immediate” meaning that you are giving it a literal value (4097). 4097 as an “upper” value becomes 0x10010000. ori is “or immediate”, with 8 being the immediate value, so … Read more
There are a few operations you can only do with DI/SI (or their extended counterparts, if you didn’t learn ASM in 1985). Among these are Which are, respectively, operations for repeated (= mass) storing, loading and scanning. What you do is you set up SI and/or DI to point at one or both operands, perhaps put a … Read more
Here is a pretty simple way to do it.
I was reading the following definition for syscall: I saw it explained that the line cmpq $-4095 %rax determines whether %rax contains a value between -1 and -4095. How does it do that? What exactly does the cmpq instruction do?
You have as input values. The instruction just subtracts 3(dec) in EDX from 100(hex) in EAX resulting in 100h-3h=0FDh.