pcjs/machines/pcx86/modules/v2/x86mods.js at master · jeffpar/pcjs · GitHub

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/**
 * @fileoverview Implements PCx86 ModRegRM decoders
 * @author Jeff Parsons <Jeff@pcjs.org>
 * @copyright © 2012-2026 Jeff Parsons
 * @license MIT <https://www.pcjs.org/LICENSE.txt>
 *
 * This file is part of PCjs, a computer emulation software project at <https://www.pcjs.org>.
 */

import X86 from "./x86.js";
import { BACKTRACK } from "./defines.js";

/**
 * Before 80386 support was added to PCx86, the approach to decoding ModRegRM bytes (which I usually
 * just call ModRM bytes) used one generated function per ModRM value.  This was optimal for 16-bit processors,
 * because the functions were small, and it was maximally efficient, turning the entire ModRM decoding operation
 * into one table lookup and function call.
 *
 * However, that approach didn't scale well for 32-bit processors, which had extended ModRM capabilities in both
 * the addressing mode dimension and the operand size dimension.  So I've rewritten ModRM decoding as 18 functions.
 * The first 9 are for 16-bit addressing modes, and the second 9 are for 32-bit addressing modes.  Within each
 * group of 9, there are 3 for 8-bit operands, 3 for 16-bit operands, and 3 for 32-bit operands.  And each group of 3
 * contains functions for register-source, memory-source, and group-source.
 *
 * Each of the 18 functions must do additional work to examine the ModRM bits, which makes decoding slightly slower,
 * but it's not really noticeable, and the speed difference didn't justify the additional generated code.  So one much
 * smaller file (x86mods.js) replaces a host of older files (x86modb.js, x86modw.js, x86modb16.js, x86modw16.js,
 * x86modb32.js, x86modw32.js, and x86modsib.js).
 *
 * You can dig up the older files from the repository if you're curious, or you can run /tools/misc/x86gen.js to
 * get a sense of what they contained (x86gen.js created most of the code, but it still had to be massaged afterward).
 */

/**
 * modRegByte16(fn)
 *
 * @this {CPUx86}
 * @param {function(number,number)} fn (dst,src)
 */
X86.modRegByte16 = function(fn)
{
    let dst, src;
    let bModRM = (this.bModRM = this.getIPByte()) & 0xC7;

    switch(bModRM) {
    case 0x00:
        src = this.getEAByteData(this.regEBX + this.regESI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndex;
        break;
    case 0x01:
        src = this.getEAByteData(this.regEBX + this.regEDI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexExtra;
        break;
    case 0x02:
        src = this.getEAByteStack(this.regEBP + this.regESI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexExtra;
        break;
    case 0x03:
        src = this.getEAByteStack(this.regEBP + this.regEDI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndex;
        break;
    case 0x04:
        src = this.getEAByteData(this.regESI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x05:
        src = this.getEAByteData(this.regEDI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x06:
        src = this.getEAByteData(this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesDisp;
        break;
    case 0x07:
        src = this.getEAByteData(this.regEBX);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x40:
        src = this.getEAByteData(this.regEBX + this.regESI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x41:
        src = this.getEAByteData(this.regEBX + this.regEDI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x42:
        src = this.getEAByteStack(this.regEBP + this.regESI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x43:
        src = this.getEAByteStack(this.regEBP + this.regEDI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x44:
        src = this.getEAByteData(this.regESI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x45:
        src = this.getEAByteData(this.regEDI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x46:
        src = this.getEAByteStack(this.regEBP + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x47:
        src = this.getEAByteData(this.regEBX + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x80:
        src = this.getEAByteData(this.regEBX + this.regESI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x81:
        src = this.getEAByteData(this.regEBX + this.regEDI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x82:
        src = this.getEAByteStack(this.regEBP + this.regESI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x83:
        src = this.getEAByteStack(this.regEBP + this.regEDI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x84:
        src = this.getEAByteData(this.regESI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x85:
        src = this.getEAByteData(this.regEDI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x86:
        src = this.getEAByteStack(this.regEBP + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x87:
        src = this.getEAByteData(this.regEBX + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0xC0:
        src = this.regEAX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiAL;
        break;
    case 0xC1:
        src = this.regECX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiCL;
        break;
    case 0xC2:
        src = this.regEDX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiDL;
        break;
    case 0xC3:
        src = this.regEBX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiBL;
        break;
    case 0xC4:
        src = (this.regEAX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiAH;
        break;
    case 0xC5:
        src = (this.regECX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiCH;
        break;
    case 0xC6:
        src = (this.regEDX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiDH;
        break;
    case 0xC7:
        src = (this.regEBX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiBH;
        break;
    default:
        src = 0;
        this.assert(false, "modRegByte16(): unrecognized modrm byte: %#04X", bModRM);
        break;
    }

    let reg = (this.bModRM >> 3) & 0x7;

    switch(reg) {
    case 0x0:
        dst = this.regEAX & 0xff;
        break;
    case 0x1:
        dst = this.regECX & 0xff;
        break;
    case 0x2:
        dst = this.regEDX & 0xff;
        break;
    case 0x3:
        dst = this.regEBX & 0xff;
        break;
    case 0x4:
        dst = (this.regEAX >> 8) & 0xff;
        break;
    case 0x5:
        dst = (this.regECX >> 8) & 0xff;
        break;
    case 0x6:
        dst = (this.regEDX >> 8) & 0xff;
        break;
    case 0x7:
        dst = (this.regEBX >> 8) & 0xff;
        break;
    default:
        dst = 0;
        break;
    }

    let b = fn.call(this, dst, src);

    switch(reg) {
    case 0x0:
        this.regEAX = (this.regEAX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiAL = this.backTrack.btiEALo;
        break;
    case 0x1:
        this.regECX = (this.regECX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiCL = this.backTrack.btiEALo;
        break;
    case 0x2:
        this.regEDX = (this.regEDX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiDL = this.backTrack.btiEALo;
        break;
    case 0x3:
        this.regEBX = (this.regEBX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiBL = this.backTrack.btiEALo;
        break;
    case 0x4:
        this.regEAX = (this.regEAX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiAH = this.backTrack.btiEALo;
        break;
    case 0x5:
        this.regECX = (this.regECX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiCH = this.backTrack.btiEALo;
        break;
    case 0x6:
        this.regEDX = (this.regEDX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiDH = this.backTrack.btiEALo;
        break;
    case 0x7:
        this.regEBX = (this.regEBX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiBH = this.backTrack.btiEALo;
        break;
    }
};

/**
 * modMemByte16(fn)
 *
 * @this {CPUx86}
 * @param {function(number,number)} fn (dst,src)
 */
X86.modMemByte16 = function(fn)
{
    let dst, src;
    let bModRM = (this.bModRM = this.getIPByte()) & 0xC7;

    switch(bModRM) {
    case 0x00:
        dst = this.getEAByteData(this.regEBX + this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x01:
        dst = this.getEAByteData(this.regEBX + this.regEDI);
        this.regEAWrite = this.regEA;
        break;
    case 0x02:
        dst = this.getEAByteStack(this.regEBP + this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x03:
        dst = this.getEAByteStack(this.regEBP + this.regEDI);
        this.regEAWrite = this.regEA;
        break;
    case 0x04:
        dst = this.getEAByteData(this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x05:
        dst = this.getEAByteData(this.regEDI);
        this.regEAWrite = this.regEA;
        break;
    case 0x06:
        dst = this.getEAByteData(this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x07:
        dst = this.getEAByteData(this.regEBX);
        this.regEAWrite = this.regEA;
        break;
    case 0x40:
        dst = this.getEAByteData(this.regEBX + this.regESI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x41:
        dst = this.getEAByteData(this.regEBX + this.regEDI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x42:
        dst = this.getEAByteStack(this.regEBP + this.regESI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x43:
        dst = this.getEAByteStack(this.regEBP + this.regEDI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x44:
        dst = this.getEAByteData(this.regESI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x45:
        dst = this.getEAByteData(this.regEDI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x46:
        dst = this.getEAByteStack(this.regEBP + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x47:
        dst = this.getEAByteData(this.regEBX + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x80:
        dst = this.getEAByteData(this.regEBX + this.regESI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x81:
        dst = this.getEAByteData(this.regEBX + this.regEDI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x82:
        dst = this.getEAByteStack(this.regEBP + this.regESI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x83:
        dst = this.getEAByteStack(this.regEBP + this.regEDI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x84:
        dst = this.getEAByteData(this.regESI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x85:
        dst = this.getEAByteData(this.regEDI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x86:
        dst = this.getEAByteStack(this.regEBP + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x87:
        dst = this.getEAByteData(this.regEBX + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0xC0:
        dst = this.regEAX & 0xff;
        break;
    case 0xC1:
        dst = this.regECX & 0xff;
        break;
    case 0xC2:
        dst = this.regEDX & 0xff;
        break;
    case 0xC3:
        dst = this.regEBX & 0xff;
        break;
    case 0xC4:
        dst = (this.regEAX >> 8) & 0xff;
        break;
    case 0xC5:
        dst = (this.regECX >> 8) & 0xff;
        break;
    case 0xC6:
        dst = (this.regEDX >> 8) & 0xff;
        break;
    case 0xC7:
        dst = (this.regEBX >> 8) & 0xff;
        break;
    default:
        dst = 0;
        this.assert(false, "modMemByte16(): unrecognized modrm byte: %#04X", bModRM);
        break;
    }

    let reg = (this.bModRM >> 3) & 0x7;

    switch(reg) {
    case 0x0:
        src = this.regEAX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiAL;
        break;
    case 0x1:
        src = this.regECX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiCL;
        break;
    case 0x2:
        src = this.regEDX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiDL;
        break;
    case 0x3:
        src = this.regEBX & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiBL;
        break;
    case 0x4:
        src = (this.regEAX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiAH;
        break;
    case 0x5:
        src = (this.regECX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiCH;
        break;
    case 0x6:
        src = (this.regEDX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiDH;
        break;
    case 0x7:
        src = (this.regEBX >> 8) & 0xff;
        if (BACKTRACK) this.backTrack.btiEALo = this.backTrack.btiBH;
        break;
    default:
        src = 0;
        break;
    }

    let b = fn.call(this, dst, src);

    switch(bModRM) {
    case 0x00:
    case 0x03:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndex;
        break;
    case 0x01:
    case 0x02:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexExtra;
        break;
    case 0x04:
    case 0x05:
    case 0x07:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x06:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesDisp;
        break;
    case 0x40:
    case 0x43:
    case 0x80:
    case 0x83:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x41:
    case 0x42:
    case 0x81:
    case 0x82:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x44:
    case 0x45:
    case 0x46:
    case 0x47:
    case 0x84:
    case 0x85:
    case 0x86:
    case 0x87:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0xC0:
        this.regEAX = (this.regEAX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiAL = this.backTrack.btiEALo;
        break;
    case 0xC1:
        this.regECX = (this.regECX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiCL = this.backTrack.btiEALo;
        break;
    case 0xC2:
        this.regEDX = (this.regEDX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiDL = this.backTrack.btiEALo;
        break;
    case 0xC3:
        this.regEBX = (this.regEBX & ~0xff) | b;
        if (BACKTRACK) this.backTrack.btiBL = this.backTrack.btiEALo;
        break;
    case 0xC4:
        this.regEAX = (this.regEAX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiAH = this.backTrack.btiEALo;
        break;
    case 0xC5:
        this.regECX = (this.regECX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiCH = this.backTrack.btiEALo;
        break;
    case 0xC6:
        this.regEDX = (this.regEDX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiDH = this.backTrack.btiEALo;
        break;
    case 0xC7:
        this.regEBX = (this.regEBX & ~0xff00) | (b << 8);
        if (BACKTRACK) this.backTrack.btiBH = this.backTrack.btiEALo;
        break;
    default:
        this.assert(false, "modMemByte16(): unrecognized modrm byte: %#04X", bModRM);
        break;
    }
};

/**
 * modGrpByte16(afnGrp, fnSrc)
 *
 * @this {CPUx86}
 * @param {Array.<function(number,number)>} afnGrp
 * @param {function()} fnSrc
 */
X86.modGrpByte16 = function(afnGrp, fnSrc)
{
    let dst;
    let bModRM = (this.bModRM = this.getIPByte()) & 0xC7;

    switch(bModRM) {
    case 0x00:
        dst = this.getEAByteData(this.regEBX + this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x01:
        dst = this.getEAByteData(this.regEBX + this.regEDI);
        this.regEAWrite = this.regEA;
        break;
    case 0x02:
        dst = this.getEAByteStack(this.regEBP + this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x03:
        dst = this.getEAByteStack(this.regEBP + this.regEDI);
        this.regEAWrite = this.regEA;
        break;
    case 0x04:
        dst = this.getEAByteData(this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x05:
        dst = this.getEAByteData(this.regEDI);
        this.regEAWrite = this.regEA;
        break;
    case 0x06:
        dst = this.getEAByteData(this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x07:
        dst = this.getEAByteData(this.regEBX);
        this.regEAWrite = this.regEA;
        break;
    case 0x40:
        dst = this.getEAByteData(this.regEBX + this.regESI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x41:
        dst = this.getEAByteData(this.regEBX + this.regEDI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x42:
        dst = this.getEAByteStack(this.regEBP + this.regESI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x43:
        dst = this.getEAByteStack(this.regEBP + this.regEDI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x44:
        dst = this.getEAByteData(this.regESI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x45:
        dst = this.getEAByteData(this.regEDI + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x46:
        dst = this.getEAByteStack(this.regEBP + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x47:
        dst = this.getEAByteData(this.regEBX + this.getIPDisp());
        this.regEAWrite = this.regEA;
        break;
    case 0x80:
        dst = this.getEAByteData(this.regEBX + this.regESI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x81:
        dst = this.getEAByteData(this.regEBX + this.regEDI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x82:
        dst = this.getEAByteStack(this.regEBP + this.regESI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x83:
        dst = this.getEAByteStack(this.regEBP + this.regEDI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x84:
        dst = this.getEAByteData(this.regESI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x85:
        dst = this.getEAByteData(this.regEDI + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x86:
        dst = this.getEAByteStack(this.regEBP + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0x87:
        dst = this.getEAByteData(this.regEBX + this.getIPAddr());
        this.regEAWrite = this.regEA;
        break;
    case 0xC0:
        dst = this.regEAX & 0xff;
        break;
    case 0xC1:
        dst = this.regECX & 0xff;
        break;
    case 0xC2:
        dst = this.regEDX & 0xff;
        break;
    case 0xC3:
        dst = this.regEBX & 0xff;
        break;
    case 0xC4:
        dst = (this.regEAX >> 8) & 0xff;
        break;
    case 0xC5:
        dst = (this.regECX >> 8) & 0xff;
        break;
    case 0xC6:
        dst = (this.regEDX >> 8) & 0xff;
        break;
    case 0xC7:
        dst = (this.regEBX >> 8) & 0xff;
        break;
    default:
        dst = 0;
        this.assert(false, "modGrpByte16(): unrecognized modrm byte: %#04X", bModRM);
        break;
    }

    let reg = (this.bModRM >> 3) & 0x7;

    let b = afnGrp[reg].call(this, dst, fnSrc.call(this));

    switch(bModRM) {
    case 0x00:
    case 0x03:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndex;
        break;
    case 0x01:
    case 0x02:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexExtra;
        break;
    case 0x04:
    case 0x05:
    case 0x07:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x06:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesDisp;
        break;
    case 0x40:
    case 0x43:
    case 0x80:
    case 0x83:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x41:
    case 0x42:
    case 0x81:
    case 0x82:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x44:
    case 0x45:
    case 0x46:
    case 0x47:
    case 0x84:
    case 0x85:
    case 0x86:
    case 0x87:
        this.setEAByte(b);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0xC0:
        this.regEAX = (this.regEAX & ~0xff) | b;
        break;
    case 0xC1:
        this.regECX = (this.regECX & ~0xff) | b;
        break;
    case 0xC2:
        this.regEDX = (this.regEDX & ~0xff) | b;
        break;
    case 0xC3:
        this.regEBX = (this.regEBX & ~0xff) | b;
        break;
    case 0xC4:
        this.regEAX = (this.regEAX & ~0xff00) | (b << 8);
        break;
    case 0xC5:
        this.regECX = (this.regECX & ~0xff00) | (b << 8);
        break;
    case 0xC6:
        this.regEDX = (this.regEDX & ~0xff00) | (b << 8);
        break;
    case 0xC7:
        this.regEBX = (this.regEBX & ~0xff00) | (b << 8);
        break;
    }
};

/**
 * modRegShort16(fn)
 *
 * @this {CPUx86}
 * @param {function(number,number)} fn (dst,src)
 */
X86.modRegShort16 = function(fn)
{
    let dst, src;
    let bModRM = (this.bModRM = this.getIPByte()) & 0xC7;

    switch(bModRM) {
    case 0x00:
        src = this.getEAShortData(this.regEBX + this.regESI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndex;
        break;
    case 0x01:
        src = this.getEAShortData(this.regEBX + this.regEDI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexExtra;
        break;
    case 0x02:
        src = this.getEAShortStack(this.regEBP + this.regESI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexExtra;
        break;
    case 0x03:
        src = this.getEAShortStack(this.regEBP + this.regEDI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndex;
        break;
    case 0x04:
        src = this.getEAShortData(this.regESI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x05:
        src = this.getEAShortData(this.regEDI);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x06:
        src = this.getEAShortData(this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesDisp;
        break;
    case 0x07:
        src = this.getEAShortData(this.regEBX);
        this.nStepCycles -= this.cycleCounts.nEACyclesBase;
        break;
    case 0x40:
        src = this.getEAShortData(this.regEBX + this.regESI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x41:
        src = this.getEAShortData(this.regEBX + this.regEDI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x42:
        src = this.getEAShortStack(this.regEBP + this.regESI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x43:
        src = this.getEAShortStack(this.regEBP + this.regEDI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x44:
        src = this.getEAShortData(this.regESI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x45:
        src = this.getEAShortData(this.regEDI + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x46:
        src = this.getEAShortStack(this.regEBP + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x47:
        src = this.getEAShortData(this.regEBX + this.getIPDisp());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x80:
        src = this.getEAShortData(this.regEBX + this.regESI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x81:
        src = this.getEAShortData(this.regEBX + this.regEDI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x82:
        src = this.getEAShortStack(this.regEBP + this.regESI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDispExtra;
        break;
    case 0x83:
        src = this.getEAShortStack(this.regEBP + this.regEDI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseIndexDisp;
        break;
    case 0x84:
        src = this.getEAShortData(this.regESI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x85:
        src = this.getEAShortData(this.regEDI + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x86:
        src = this.getEAShortStack(this.regEBP + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0x87:
        src = this.getEAShortData(this.regEBX + this.getIPAddr());
        this.nStepCycles -= this.cycleCounts.nEACyclesBaseDisp;
        break;
    case 0xC0:
        src = this.regEAX & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = this.backTrack.btiAL; this.backTrack.btiEAHi = this.backTrack.btiAH;
        }
        break;
    case 0xC1:
        src = this.regECX & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = this.backTrack.btiCL; this.backTrack.btiEAHi = this.backTrack.btiCH;
        }
        break;
    case 0xC2:
        src = this.regEDX & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = this.backTrack.btiDL; this.backTrack.btiEAHi = this.backTrack.btiDH;
        }
        break;
    case 0xC3:
        src = this.regEBX & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = this.backTrack.btiBL; this.backTrack.btiEAHi = this.backTrack.btiBH;
        }
        break;
    case 0xC4:
        src = this.getSP() & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = X86.BTINFO.SP_LO; this.backTrack.btiEAHi = X86.BTINFO.SP_HI;
        }
        break;
    case 0xC5:
        src = this.regEBP & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = this.backTrack.btiBPLo; this.backTrack.btiEAHi = this.backTrack.btiBPHi;
        }
        break;
    case 0xC6:
        src = this.regESI & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = this.backTrack.btiSILo; this.backTrack.btiEAHi = this.backTrack.btiSIHi;
        }
        break;
    case 0xC7:
        src = this.regEDI & 0xffff;
        if (BACKTRACK) {
            this.backTrack.btiEALo = this.backTrack.btiDILo; this.backTrack.btiEAHi = this.backTrack.btiDIHi;
        }
        break;
    default:
        src = 0;
        this.assert(false, "modRegShort16(): unrecognized modrm byte: %#04X", bModRM);
        break;
    }

    let reg = (this.bModRM >> 3) & 0x7;

    switch(reg) {
    case 0x0:
        dst = this.regEAX & 0xffff;
        break;
    case 0x1:
        dst = this.regECX & 0xffff;
        break;
    case 0x2:
        dst = this.regEDX & 0xffff;
        break;
    case 0x3:
        dst = this.regEBX & 0xffff;
        break;
    case 0x4:
        dst = this.getSP() & 0xffff;
        break;
    case 0x5:
        dst = this.regEBP & 0xffff;
        break;
    case 0x6:
        dst = this.regESI & 0xffff;
        break;
    case 0x7:
        dst = this.regEDI & 0xffff;
        break;
    default:
        dst = 0;
        break;
    }

    let w = fn.call(this, dst, src);
    this.assert(!(w & ~0xffff), "modRegShort16(): result out of range: %#010X", w);

    switch(reg) {
    case 0x0:
        this.regEAX = (this.regEAX & ~0xffff) | w;
        if (BACKTRACK) {
            this.backTrack.btiAL = this.backTrack.btiEALo; this.backTrack.btiAH = this.backTrack.btiEAHi;
        }
        break;
    case 0x1:
        this.regECX = (this.regECX & ~0xffff) | w;
        if (BACKTRACK) {
            this.backTrack.btiCL = this.backTrack.btiEALo; this.backTrack.btiCH = this.backTrack.btiEAHi;
        }
        break;
    case 0x2:
        this.regEDX = (this.regEDX & ~0xffff) | w;
        if (BACKTRACK) {
            this.backTrack.btiDL = this.backTrack.btiEALo; this.backTrack.btiDH = this.backTrack.btiEAHi;
        }
        break;
    case 0x3:
        this.regEBX = (this.regEBX & ~0xffff) | w;
        if (BACKTRACK) {
            this.backTrack.btiBL = this.backTrack.btiEALo; this.backTrack.btiBH = this.backTrack.btiEAHi;
        }
        break;
    case 0x4:
        this.setSP((this.getSP() & ~0xffff) | w);
        break;
    case 0x5:
        this.regEBP = (this.regEBP & ~0xffff) | w;
        if (BACKTRACK) {
            this.backTrack.btiBPLo = this.backTrack.btiEALo; this.backTrack.btiBPHi = this.backTrack.btiEAHi;
        }
        break;
    case 0x6:
        this.regESI = (this.regESI & ~0xffff) | w;
        if (BACKTRACK) {
            this.backTrack.btiSILo = this.backTrack.btiEALo; this.backTrack.btiSIHi = this.backTrack.btiEAHi;
        }
        break;
    case 0x7:
        this.regEDI = (this.regEDI & ~0xffff) | w;
        if (BACKTRACK) {
            this.backTrack.btiDILo = this.backTrack.btiEALo; this.backTrack.btiDIHi = this.backTrack.btiEAHi;
        }
        break;
    }
};

/**
 * modMemShort16(fn)
 *
 * @this {CPUx86}
 * @param {function(number,number)} fn (dst,src)
 */
X86.modMemShort16 = function(fn)
{
    let dst, src;
    let bModRM = (this.bModRM = this.getIPByte()) & 0xC7;

    switch(bModRM) {
    case 0x00:
        dst = this.getEAShortData(this.regEBX + this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x01:
        dst = this.getEAShortData(this.regEBX + this.regEDI);
        this.regEAWrite = this.regEA;
        break;
    case 0x02:
        dst = this.getEAShortStack(this.regEBP + this.regESI);
        this.regEAWrite = this.regEA;
        break;
    case 0x03: