my current solution for modifying games more or less safe is using dosbox as a backend
for example: im able to hook function calls and overwrite code parts, very good for porting because
you can port a function while the function is in use by the emulated code
for example the data compression routine of the Alpha Waves game
disassembled in IDA then converter to my tiny "emulator" that fakes the minimal
aspects of the x86 code to ease the porting to C
emu_t just got some registers, memory and methods that look like the original
asm and behave like the original asm code - but its just C/C++ code
this function gets called by dosbox when the emulated code actually wanted to call the original
16bit code, i can debug, step through it, log data, write unit-tests etc.
this is my third try to port that function properbly - before just in assembler and 16bit C
but subtile micro difference seemed to work but my port was until now only working with 95%
of the data
for example: im able to hook function calls and overwrite code parts, very good for porting because
you can port a function while the function is in use by the emulated code
for example the data compression routine of the Alpha Waves game
disassembled in IDA then converter to my tiny "emulator" that fakes the minimal
aspects of the x86 code to ease the porting to C
emu_t just got some registers, memory and methods that look like the original
asm and behave like the original asm code - but its just C/C++ code
this function gets called by dosbox when the emulated code actually wanted to call the original
16bit code, i can debug, step through it, log data, write unit-tests etc.
this is my third try to port that function properbly - before just in assembler and 16bit C
but subtile micro difference seemed to work but my port was until now only working with 95%
of the data
Code Select
void UNCOMPRESS_sub_1BAE7(emu_t &e)
{
start:
e.push(e.es);
e.push(e.di);
e.cx = 0x80;
e.ax = e.ds;
e.es = e.ax;
e.di = 0x301;
e.xor(e.ax, e.ax);
e.rep_stosw();
e.pop(e.di);
e.pop(e.es);
e.sub(e.di, *e.word_ptr(e.cs, 0xBAA2));
e.ax = e.di;
e.shr(e.ax, 1);
e.shr(e.ax, 1);
e.shr(e.ax, 1);
e.shr(e.ax, 1);
e.cx = e.es;
e.add(e.cx, e.ax);
e.es = e.cx;
e.and (e.di, 0x0F);
e.add(e.di, *e.word_ptr(e.cs, 0xBAA2));
e.push(e.ds);
e.push(e.es);
e.push(e.si);
e.push(e.di);
e.cx = 4;
e.di = 0xBA9A; // offset byte_1BA9A; ???
e.ax = e.cs; // seg seg000 // cs register; ???
e.es = e.ax;
e.lds(e.si, *e.dword_ptr(e.cs, 0xBAA4));
e.ax = e.si;
e.shr(e.ax, 1);
e.shr(e.ax, 1);
e.shr(e.ax, 1);
e.shr(e.ax, 1);
e.dx = e.ds;
e.add(e.ax, e.dx);
e.ds = e.ax;
e.and (e.si, 0x0F);
*e.word_ptr(e.cs, 0xBAA4) = e.si;
*e.word_ptr(e.cs, 0xBAA4 + 2) = e.ds;
e.add(*e.word_ptr(e.cs, 0xBAA4), e.cx);
e.rep_movsb();
e.pop(e.di);
e.pop(e.si);
e.pop(e.es);
e.pop(e.ds);
e.dx = *e.word_ptr(e.cs, 0xBA9C);
e.inc(e.dx);
e.cmp(*e.byte_ptr(e.cs, 0xBA9A), 0);
if (e.jnz())
goto loc_1BB63;
goto loc_1BC52;
// ---------------------------------------------------------------------------
loc_1BB63:
e.push(e.ds);
e.push(e.es);
e.push(e.di);
e.xor (e.ch, e.ch);
e.cl = *e.byte_ptr(e.cs, 0xBA9A);
e.di = 0x201;
e.ax = e.ds;
e.es = e.ax;
e.ds = *e.word_ptr(e.cs, 0xBAA4 + 2);
e.si = *e.word_ptr(e.cs, 0xBAA4);
e.add(*e.word_ptr(e.cs, 0xBAA4), e.cx);
e.rep_movsb();
e.cl = *e.byte_ptr(e.cs, 0xBA9A);
e.xor (e.ch, e.ch);
e.di = 1;
e.add(*e.word_ptr(e.cs, 0xBAA4), e.cx);
e.rep_movsb();
e.cl = *e.byte_ptr(e.cs, 0xBA9A);
e.di = 0x101;
e.add(*e.word_ptr(e.cs, 0xBAA4), e.cx);
e.rep_movsb();
e.pop(e.di);
e.pop(e.es);
e.pop(e.ds);
e.xor (e.ch, e.ch);
e.cl = *e.byte_ptr(e.cs, 0xBA9A);
e.xor (e.ah, e.ah);
e.bx = 1;
loc_1BBB4:
e.al = *e.byte_ptr(e.ds, e.bx + 0x200);
e.si = e.ax;
e.dl = *e.byte_ptr(e.ds, e.si + 0x301);
*e.byte_ptr(e.ds, e.bx + 0x402) = e.dl;
*e.byte_ptr(e.ds, e.si + 0x301) = e.bl;
e.inc(e.bx);
if (e.loop())
goto loc_1BBB4;
e.dx = *e.word_ptr(e.cs, 0xBA9C);
e.inc(e.dx);
e.cx = 1;
loc_1BBD2:
e.dec(e.dx);
if (e.jnz())
goto loc_1BBE1;
loc_1BBD5:
e.cmp(*e.byte_ptr(e.cs, 0xBA9B), 0);
if (e.jz())
goto locret_1BBE0;
goto start;
// ---------------------------------------------------------------------------
locret_1BBE0:
return;
// ---------------------------------------------------------------------------
loc_1BBE1:
e.push(e.ds);
e.si = *e.word_ptr(e.cs, 0xBAA4 + 2);
e.ds = e.si;
e.si = *e.word_ptr(e.cs, 0xBAA4);
e.lodsb();
*e.word_ptr(e.cs, 0xBAA4) = e.si;
e.pop(e.ds);
e.bx = e.ax;
e.cmp(*e.byte_ptr(e.ds, e.bx + 0x301), 0);
if (e.jnz())
goto loc_1BC01;
e.stosb();
goto loc_1BBD2;
// ---------------------------------------------------------------------------
loc_1BC01:
e.bl = *e.byte_ptr(e.ds, e.bx + 0x301);
e.xor (e.ax, e.ax);
e.push(e.ax);
goto loc_1BC35;
// ---------------------------------------------------------------------------
loop_x:
e.bp = e.ax;
e.cmp(*e.byte_ptr(e.ds, e.bp + 0x301), 0);
if (e.jz())
goto loc_1BC44;
e.cmp(e.bl, *e.byte_ptr(e.ds, e.bp + 0x301));
if (e.ja())
goto loc_1BC30;
e.al = e.bl;
e.bl = *e.byte_ptr(e.ds, e.bp + 0x301);
loc_1BC22:
e.bl = *e.byte_ptr(e.ds, e.bx + 0x402);
e.or (e.bl, e.bl);
if (e.jz())
goto loc_1BC42;
e.cmp(e.bl, e.al);
if (e.jb())
goto loc_1BC35;
goto loc_1BC22;
// ---------------------------------------------------------------------------
loc_1BC30:
e.bl = *e.byte_ptr(e.ds, e.bp + 0x301);
loc_1BC35:
e.al = *e.byte_ptr(e.ds, e.bx + 0x100);
e.ah = e.bl;
e.push(e.ax);
e.xor (e.ah, e.ah);
e.al = *e.byte_ptr(e.ds, e.bx);
goto loop_x;
// ---------------------------------------------------------------------------
loc_1BC42:
e.ax = e.bp;
loc_1BC44:
e.stosb();
e.pop(e.ax);
e.or (e.ax, e.ax);
if (e.jnz())
goto loc_1BC4C;
goto loc_1BBD2;
// ---------------------------------------------------------------------------
loc_1BC4C:
e.bl = e.ah;
e.xor (e.ah, e.ah);
goto loop_x;
// ---------------------------------------------------------------------------
loc_1BC52:
e.push(e.ds);
e.push(e.es);
e.cx = *e.word_ptr(e.cs, 0xBA9C);
e.push(e.cx);
e.ds = *e.word_ptr(e.cs, 0xBAA4 + 2);
e.si = *e.word_ptr(e.cs, 0xBAA4);
e.add(*e.word_ptr(e.cs, 0xBAA4), e.cx);
e.rep_movsb();
e.pop(e.cx);
e.pop(e.es);
e.pop(e.ds);
goto loc_1BBD5;
}
