Gregory's ServerMost people think of machine instructions as the lowest level of software, but many processors contain microcode. An instruction is broken down into micro-instructions. The 8086 has 21-bit micro-instructions; each executes a register move (source→dest) and an action in parallel.
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 Here's what the microcode looks like for the string move instruction: the main code, a micro-subroutine to check the CX register, and a micro-subroutine to handle interrupts. Each micro-instruction has a move on the left and an action on the right. Some hardware helps the microcode. Latches remember the "repeat" prefix. Hardware lets the microcode test conditions, e.g. is CX zero, is there a repeat prefix, is the loop exit condition satisfied, is there an interrupt. Photo shows the chip circuitry for 16 conditions. Arithmetic and logic instructions use the ALU (Arithmetic/Logic Unit). But incrementing/decrementing the memory pointers doesn't use the ALU. Instead, it uses a special adder that calculates memory addresses. A "Constant ROM" holds the values (-6 to +2) that need to be added. For more about the implementation of the 8086's string instructions, including a detailed walkthrough of the microcode, see my blog post: Thanks to Andrew Jenner for disassembling the microcode. @kenshirriff Fun with MOVS: x86 supports self-modifying code, but what about self-modifying _instructions_? Turns out with MOVS you could do self-modifying instructions. @kenshirriff Thank you! I love your articles on the 8086! Just one remark: the 20 bit address bus gives access to 1 megabyte, not 4. |
A string instruction performs a loop up to 64K times. Because the loop happens inside the processor, it's faster than writing the loop in assembly. The SI register points to the source, the DI register points to the dest, and the CX register counts. The details are complicated.