get_eff_addr_modrm_16(9)

💅 get_eff_addr_modrm_16 — like, the kernel API manual, bestie

okay so real talk, this one lives in the get_eff_addr_modrm_16(9) corner of the Kernel API Manual and it's giving "niche x86 decoder lore that only the girlies in arch/x86/lib understand" ✨

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🌸 NAME 🌸

get_eff_addr_modrm_16 — bestie, it obtains the referenced effective address via ModRM 🎀 no cap, that's the whole vibe.

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✨ SYNOPSIS ✨

int get_eff_addr_modrm_16 (struct insn *insn , struct pt_regs *regs ,
int *regoff , short *eff_addr );

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🦋 ARGUMENTS 🦋

• insn — the instruction, and it must be valid, like that's not optional, that's the whole assignment 💅
• regs — the register values exactly as seen when entering kernel mode
• regoff — the obtained operand offset, in pt_regs, that's associated with the segment
• eff_addr — the obtained effective address, the main character fr fr 👑

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🔥 DESCRIPTION 🔥

okay so picture this: you want the 16-bit effective address referenced by the ModRM byte of insn, right? That's the mission. 🫶

So after identifying the registers involved in the register-indirect memory reference, its value gets obtained from the operands in regs. The computed address is stored in eff_addr. Also — and this part is cute — the register operand that indicates the associated segment is stored in regoff. You can use that parameter later to determine such segment. iykyk 😌

⭐ the returns, periodt

• 0 on success. eff_addr will have the referenced effective address. regoff will have a register, as an offset from the base of pt_regs, that can be used to resolve the associated segment. slay 💖
• -EINVAL on error. 💀

ngl bestie, that's it — no secret extra returns hiding in the back of the group chat.

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🌈 SEE ALSO 🌈

Kernel file ./arch/x86/lib/insn-eval.c — and honestly the whole friend group is here, so let's list every single one because we do not leave anyone on read 🙈:

is_string_insn(9), insn_has_rep_prefix(9), get_seg_reg_override_idx(9), check_seg_overrides(9), resolve_default_seg(9), resolve_seg_reg(9), get_segment_selector(9), get_reg_offset_16(9), get_desc(9), insn_get_seg_base(9), get_seg_limit(9), insn_get_code_seg_params(9), insn_get_modrm_rm_off(9), insn_get_modrm_reg_off(9), insn_get_modrm_reg_ptr(9), get_seg_base_limit(9), get_eff_addr_reg(9), get_eff_addr_modrm(9), get_eff_addr_sib(9), get_addr_ref_16(9), get_addr_ref_32(9), get_addr_ref_64(9), insn_get_addr_ref(9), insn_fetch_from_user(9), insn_fetch_from_user_inatomic(9), insn_decode_from_regs(9), insn_decode_mmio(9)

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okay that's the whole page and she ate, no crumbs 🍓

source: ./arch/x86/lib · dated October 2026 · and that's the get_eff_addr_modrm_16(9) sign-off, mwah 💋🧚‍♀️