![]() ![]() The original 8086 only had registers that were 16 bits in size, effectively allowing to store one value of the range (or simpler: it could address up to 65536 different bytes, or 64 kibibytes) - but the address bus (the connection to the memory controller, which receives addresses, then loads the content from the given address, and returns the data back on the data bus to the CPU) was 20 bits in size, effectively allowing to address up to 1 mebibyte of memory. Each is the lower half of the corresponding ZMM register. Each is the lower quarter of the corresponding ZMM register. 512-bit registers that are not addressable in narrower modes unless AVX-512VL is implemented. Each ZMM register includes the corresponding YMM register as its lower half. ![]() Each YMM register includes the corresponding XMM register as its lower half. XMM8~15 SSE3 and newer and AMD (but not Intel) SSE2. Some also have AVX-512, which lengthens them to 512 bits and adds 16 more registers. Most CPUs made since 2011 also have AVX, a further extension that lengthens these registers to 256 bits. There is no way to directly access bits 8–15 of those registers, as AH allows for AX.ġ28-bit, 256-bit and 512-bit (SSE/AVX) Ħ4-bit x86 includes SSE2 (an extension to 32-bit x86), which provides 128-bit registers for specific instructions. For example, the low byte of RSP can be accessed using SPL. 64-bit versions of x86 also allow the low byte of RSP, RBP, RSI, RDI to be accessed directly. Using R8 as an example, the names corresponding to those widths are R8B, R8W, and R8D, respectively. R8–R15 can be accessed as 8-bit, 16-bit, or 32-bit registers. They are counted as if the registers above are registers zero through seven, inclusively, rather than one through eight. R8–15 These are new extra registers for 64-bit. RIP This is the full 64-bit instruction pointer and should be used instead of EIP (which will be inaccurate if the address space is larger than 4 GiB, which may happen even with 4 GiB or less of RAM). RAX, RBX, RCX, RDX, RSP, RBP, RSI, RDI These are the 64-bit versions of the registers shown above. ![]() The names of the 64-bit registers are the same of those of the 32-bit registers, except beginning with an 'R'. EFLAGS An expanded version of the 16-bit FLAGS register. Always use this instead of IP on 32-bit systems. The names for the 32-bit registers are simply the 16-bit names with an 'E' prepended.ĮAX, EBX, ECX, EDX, ESP, EBP, ESI, EDI These are the 32-bit versions of the registers shown above. With the chips beginning to support a 32-bit data bus, the registers were also widened to 32 bits. FLAGS (F) A number of single-bit flags that indicate (or sometimes set) the current status of the processor. Offset from the code segment CS, this points at the instruction currently being executed. These are the offset in memory of the current code segment, data segment, stack segment and extra segment respectively. BP can then be used to find the parameters on the stack, no matter how much the stack is used in the meanwhile. Parameters for subroutines are commonly pushed onto the stack when the subroutine is called, and BP is set to the value of SP when a subroutine starts. BP The stack frame, usually treated as an offset from the stack segment SS. Data is pushed onto the stack for temporary storage, and popped off the stack when it is needed again. ![]() SP This is the stack pointer, offset usually from the stack segment SS. By default, SI is offset from the DS data segment, DI is offset from the ES extra segment, but either or both of these can be overridden. SI, DI These registers are usually used as offsets into data space. So AX = AH (high 8-bit) and AL (low 8-bit). AX, BX, CX, DX These general purpose registers can also be addressed as 8-bit registers. If you are using DOSBox or FreeDOS, you can use "debug.exe" as provided by FreeDOS. On DOS and up to 32-bit Windows, you can run a very handy program called "debug.exe" from a DOS shell, which is very useful for learning about 8086. The registers found on the 8086 and all subsequent x86 processors are the following: AX, BX, CX, DX, SP, BP, SI, DI, CS, DS, SS, ES, IP and FLAGS. ![]()
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