Or the similar PowerPC, also called addi:īoth have 16-bit immediate values: 0–65535, or two bytes. Not great when you're mostly working with 32-bit numbers and addresses.Ĭompare it also to the equivalent instruction in MIPS, addi: That doesn't give a huge range of numbers: 0–4095, or a byte and a half. If it's set to 1, then operand 2 is an immediate value. 0100 is ADD.īit 25 is the "immediate" bit. The four-bit opcode field in bits 24–21 defines exactly which instruction this is: add, subtract, move, compare, and so on. Here's the bit layout of an ARM data processing instruction:Īny instruction with bits 27 and 26 as 00 is data processing.
#Visual arm emulator examples full
However, it means that any instruction with an immediate value operand cannot represent a full 32-bit number. This is a good design decision, and it makes instruction decode and pipeline management much easier than with the variable instruction size of x86 or 680x0. Immediate value encodingĪRM, like other RISC architectures MIPS and PowerPC, has a fixed instruction size of 32 bits. This value is encoded in the instruction, so that it's immediately available. This allows for some fairly complex single-instruction operations, and more importantly lots of fun optimising your assembler code.īut the instruction I want to describe in more detail here is the third and simplest one: adding a register to a constant value. The second example shows the use of the barrel shifter, which can shift or rotate the second operand before performing the operation. The first is easy to understand: add two registers, and store in a third. For example, here are three different ADD instructions: ADD r0, r2, r3 r0 = r2 + r3 ADD r0, r2, r3, LSL #4 r0 = r2 + (r3 << 4) ADD r0, r2, #& 4F0000 r0 = r2 + 0x4F0000 All ARM processors (like the one in your iPhone, or the other dozen in various devices around your home) have 16 basic data processing instructions.Įach data processing instruction can work with several combinations of operands. Machine code is what computer processors run on: binary representations of simple instructions. Perhaps I should start with some background. Despite only using 12 bits of instruction space, the immediate value can represent a useful set of 32-bit constants. It's typical of the design of the processor architecture: elegant, pragmatic, and quirky. The ARM instruction set encodes immediate values in an unusual way.
#Visual arm emulator examples software
ARM immediate value encoding Alisdair McDiarmid is a senior software engineer based in Toronto, Ontario.