We check that reg32 agrees with the required size of the instruction from the original database (it does). The register has a number, which is specified in another data file: eax REG_EAX reg32 0 We have a single such operand, a register. The modr/m byte is used to encode registers or indirect memory references. This specifies some extra bits that we will need in the modr/m bytem, and causes us to generate it. If we were producing 16-bit output, we'd produce the prefix now ( 0圆6), but I'll assume we aren't and carry on. This tells us that if we're assembling code for a 16-bit output format, the instruction needs an operand-size override prefix. This identifies how we handle the "lock" prefix. The mi is a descriptiuon of the operands: one a modr/m (register or memory) operand (which means we'll need to append a modr/m byte to the end of the instruction, which we'll come to later) and one an immediate instruction (which will be used in the description of the instruction).
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This is a set of instructions that describe how to generate the machine code instruction that's required: Once we've got the line from the database, we look at the third column, which for this instruction is: If you don't find a match, that's an error that needs to be presented to the user ("illegal combination of opcode and operands" or similar is the usual text). Once you have this structure, you run through the instruction database and find the line that matches both the instruction name and the types of the operands.
For the instruction above, this would probably result in a structure that contains the instruction, ADD, and an array of operands (a reference to the register EAX and the value 42). Now, you need to take your text input and parse it into individual instructions and operands. An example assembly instruction that would use this version is this: add eax, 42 a constant directly included in the instruction). There are multiple variants of this instruction, and the specific one that is being described here is the variant that takes either a 32-bit register or memory address and adds an immediate 8-bit value (i.e. What this means is that it describes the instruction ADD. Lets pick an arbitrary line from the database: ADD rm32,imm8 386,LOCK This is the instruction database for x86 processors as used by the NASM assembler (which I helped write, although not the parts that actually translate instructions). The first thing you need is something like this file.
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