Saturday, January 26, 2013

Reverse engineering and other stuff.

IRA the dissembler again

Wow, it has started to piss me off, well not rally, what i considered as relative easy thing to do is not as easy as first thought.

The dissembler does good job at generating Assembly code, but does poor job as knowing the different between strings and assembly code, the challenge was not Assembly but cleaning up after a tornado.

What the dissembler generated:

     BVS.S    LAB_001C
     MOVEQ    #117,D2
     BVS.S    LAB_001F
     DC.W    $696f
     BGT.S    LAB_0018
     DC.W    $6c69
     BHI.S    LAB_0020
     BSR.S    LAB_0021
     DC.W    $7900
     BEQ.S    LAB_0022
     BSR.S    LAB_0022
     DC.W    $6869
     DC.W    $6373
     MOVEA.L    26978(A4),A7
     MOVEQ    #97,D1
     MOVEQ    #121,D1

This should be:

     DC.b        "intuition.library",0
     DC.b        "graphics.library",0

You can see that some thing went wrong when you see DC.w and DC.b mixed whit assembly its not common, maybe if it was a undocumented machine code instruction.

Just to repeat my self, I know some you who read this might not have written assembly so explain it again, DC.b is for arrays of byte and string (array of bytes that has ascii values), DC.w is for 16bit Integer (WORD) of arrays.

“DS” is for size of data and reserves chunk inside your code.

What you really need to use to clean it up a good hex editor, so you can look inside the exe file, and see what text strings should be.

I write a few commands to help me find hex values.

hex_to_string and string_to_hex, it's nice to have if you wonder if some thing really is ASCII and not numbers.


When debugging code one of my favorite tools is PrintF, simply because debuggers don't work so well under AmigaOS, we have grim repaper that displays power pc registers and 680x0 emulated registers, and where it crashed, but 680x0 code its translated as program runs so its hard to know where it crashed, and also grim only displays crash location as powerpc assembly.

Under UAE there are probably better tools, but I need to find the crashes under AmigaOS4, not under UAE, so not that useful,


C vs Assembly language

 Sorry I just don't get it, way are people (Franko) telling me that Assembly is easy language?

This window display C code that does the Dos.library / PrintF command just as Assembly code above.

Well the code lies, I should have opened the DOS.library but its no longer necessary under AmigaOS4, whit -lauto option, so not a big lie, it works as its written.

but as you can see Assembly version of printf takes up to 7 lines to do the same as C does in just ONE single line, and it does the same thing.

And also you can see that strings has to be put some where else, and then you need to move the values in to ARGS array (D2), before command is executed, it just allot of more work.

Well maybe Assembly is not that complicated, but does require a lot more work, in the old days it made sense to do it in Assembly because you needed to optimize for speed as CPU's back then was slow, and you need to optimize for size as storage space was critical, but today it makes no sense to do it unless your optimizing something critical.

Wednesday, January 16, 2013

Reverse engendering of 680x0 assembly code.

Trying to make sense of it all, IRA the disassembler has been really help full to gives some clues; by providing EQU constants for hardware registers, and some other stuff I don't know what is.

It is worth to point out that any access to hardware registers are illegal under AmigaOS4.1, unless your running on old hardware, so we need to replace it whit system friendly code, so we need to look for any references to this constants, that I have marked, and replace the code.

Next thing that is a issue is to understand what the code does, there are some clues, see the lines that starts whit JSR, JSR is short for Jump To Sub Routine. In front of A6 (address register 6), you have a negative value, this is a offset value we call LVO, A6 is loaded whit library base address, we just need to find what library and compare the LVO values to that library offsets, LVO number are not unique to one library they can be the same for number of libraries, so we can't just auto replace the values whit constants.

Another thing that will help making it possible to understand some thing is to look at bottom of the code, this where you find data, like strings.

DC and DS is defines data space, DC is for values you enter, while DS just reserves chunk of space, it is the DC that we are most interested, in front of DC there is label it represent a reference to the data, the label has been generated by IRA disassembler and not human understandable, we need to replace the label name whit some thing we understand, so we replace LAB_0F71 whit LAB_STERO_LEVEL, and LAB_0F6B whit LAB_VOLUME_BOOST, we do that for all readable strings, we most be careful to replace every reference of LAB_0F6B and LAB_0F71.

Saturday, January 12, 2013

Paula sound, so where does that 14bit audio come from?

So where does that 14bit stereo sound come from? You might have wondered, well after reading the hardware reference manual, it explains that 4 audio channels can be combined in to two.

In this mode, one of channels controls the volume (a value from 0 to 63) 6bits, and wave from is (127 to -127) 8bit, so if you add that up 6+8 = 14bit.

This not a hack, that's a urban myth, it was designed to be like this. Its properly more complicated to calculate wave form and volume, and also it does not take advantage of all bits in the audio channels, plus it does need a bit more CPU if I'm not mistaken.

Its a shame they did not provided a proper 16bit D/A converter instead, anyway the sound was not that bad compared some early sound blaster sound cards that sounded a bit sour.

I think this graph illustrates the dilemma, where audio quality is better at lower amplitude, then on higher, as as the amplitude increases, because the 8bits are more compressed at lower volume. So you do not get popper 14bit, where the bits are evenly spread out, what you get if you made a sinus or a sawtooth, you see that top and bottom of sinus are more pixelated then in the center of sinus.

I have also played whit Paula whit some help, few small errors, and this codes players a beep. I have not really found out about interrupts, the DMA is supposed to trigger a interrupt when sound was played, so you fill the buffer whit new sound, but I can't find any interrupt vector to configure in the hardware reference manual.


OldOpenLibrary EQU -$0198
WriteChars EQU -$03AE
CloseLibrary EQU -$019E
Delay EQU -198

FREEMEM        EQU    -210

CUSTOM        EQU    $DFF000
AUD0LCH        EQU    $0A0
AUD0LCL        EQU $0A2
AUD0LEN        EQU $0A4
AUD0VOL        EQU    $0A8
AUD0PER        EQU    $0A6
DMACON        EQU $096


        LEA DOS(pc),A1
        MOVE.l #0,D0
        MOVE.l EXECBASE,a6
        jsr     OldOpenLibrary(a6)
        move.l  d0,DOSBASE
        beq.s   .Out

        move.l  #msg_start,d1
        moveq  #9,d2
        JSR .Write

        ; Alloc Sound wave
        MOVE.l #100,D0    ; size
        MOVE.l #2,D1        ; chip mem
        MOVE.l EXECBASE,a6
        JSR ALLOCMEM(a6)
        MOVE.l d0,SINDATA
        BEQ.s    .NOMEM
        ; Setup sound wave
        MOVE.L SINDATA,a1
        MOVE.b 0,0(a1)
        MOVE.b 90,1(a1)
        MOVE.b 127,2(a1)
        MOVE.b 90,3(a1)
        MOVE.b 0,4(a1)
        MOVE.b -90,5(a1)
        MOVE.b -127,6(a1)
        MOVE.b -90,7(a1)

        ; Play sound
        LEA.l CUSTOM,a0
        MOVE.l SINDATA,AUD0LCH(a0)  ; sound wave to play
        MOVE.W #4,AUD0LEN(a0) ; length of sound wave
        MOVE.W #64,AUD0VOL(a0) ; sound volume
        MOVE.W #447,AUD0PER(a0) ; set audio period
        ; enable (bit 15) dma (bit 9), audio channel 0 (bit 0)
        MOVE.W #$8201,DMACON(a0)

        ; Wait for sound to be played
        move.l DOSBASE,A6
        move.l #40,D1
        JSR Delay(A6)

        LEA.l CUSTOM,a0
        MOVE.W #1,AUD0VOL(a0) ; sound volume
        MOVE.W #0,AUD0LEN(a0) ; length of sound wave
        MOVE.W #220,AUD0PER(a0) ; set audio period
        MOVE.W #$0001,DMACON(a0)

        move.l  #msg_step,d1
        moveq  #7,d2
        JSR .Write

        ; Wait for sound to be played
        move.l DOSBASE,A6
        move.l #20,D1
        JSR Delay(A6)

        ; Free sound wave
        MOVE.l #100,D0
        MOVE.l SINDATA,a1
        MOVE.l EXECBASE,a6
        JSR FREEMEM(a6)

        move.l  #msg_ok,d1
        moveq  #7,d2
        JSR .Write
        JMP .Out      
        move.l  #msg_nomem,d1
        moveq  #13,d2
        JSR .Write
        JMP .Out      
        move.l DOSBASE,A6
        jsr     WriteChars(a6)
        move.l  DOSBASE,a1
        move.l  EXECBASE,a6
        jsr     CloseLibrary(a6)
        dc.b    "dos.library",0
        dc.l 0
        DC.L    0
        dc.b   "START!!!",$A,0
        dc.b    "STEP!!",$A,0
        dc.b    "OK!!!!",$A,0
        dc.b    "AllocMem failed, no memory!",$A,0