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> DOS > Accessing memory at 0xF000FFFE (computer type) in fasm DOS |
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revolution 17 Feb 2013, 19:57
You probably need to consider your segment register value. Does DS=0x0000 ?
And also for 32-bit addresses in DOS you need to be running in FRM or Unreal mode else you will get an exception generated. |
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17 Feb 2013, 19:57 |
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deathmood 17 Feb 2013, 20:06
No Ds is not zero (I looked in turbo debugger). How can run unreal mode?
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17 Feb 2013, 20:06 |
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revolution 17 Feb 2013, 20:09
You could also try:
Code: mov ax,0xf000 mov ds,ax mov al,[0xfffe] |
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17 Feb 2013, 20:09 |
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deathmood 17 Feb 2013, 20:15
Wow! That really worked for me!!! Can you post explanation? Please...
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17 Feb 2013, 20:15 |
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baldr 17 Feb 2013, 20:53
deathmood,
Consult Intel SDM or AMD APM about real-address mode and its translation of logical addresses. Those segment*16+offset thingies. |
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17 Feb 2013, 20:53 |
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shutdownall 17 Feb 2013, 22:58
deathmood wrote: Wow! That really worked for me!!! Can you post explanation? Please... Under DOS you can address only 1 MByte. It is used in 16 bit mode (use16) with segment register. The register and corresponding segment register are simply added. What are you looking for is address F:FFFE and not F000:FFFE. Segment register is moved 4 bit to left and added with register. So DS=F000 and AX=FFFE is same as DS=FF00 and AX=FFE or DS=FFF0 and AX=FE or DS=FFFF and AX=E The default segment register is DS for normal registers, SS for SP and CS for IP. You can use other segment registers explicitly: Code: mov ax,0xf000 mov es,ax mov al,es:[0xfffe] |
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17 Feb 2013, 22:58 |
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freecrac 18 Feb 2013, 08:34
shutdownall wrote:
But under a real DOS we can switch to the 16 bit PM, or to the 16 bit Unrealmode/16 bit FRM for to address up to 4GB(80386+) also using the 16 bit mode (use16). Because the number(32 or 16 of the assembler directive of the addressmode (example "[use16]" and "[use32]") does not refers to the maximum amount of memory that the application can address! (The maximum amount of memory that our application can be address is refering to the segment entries of the segmentsize inside of a GDT/LDT selector, or by the default size for a segment of 64 kb.) Starting with the 80386 we can use operandsize- and addresssize- override prefixes. Those prefixes can be used in combination with the 16 bit address mode and with the 32 bit address mode. Additional it can be used with the real addressmode and with the protected mode and the virtual 86 mode. Those prefixes reverse the default operandsize and/or the addresssize for one instruction in the codesegment. The default operandsize and the addresssize is specified by the D flag in the code-segment descriptor(or if there is no GDT/LDT, then we become the 16 bit addressmode after the POST-process of the bios is done.) The only one differnce between the 32 bit - and the 16 bit addressmode is the meaning and the usage of those operandsize- and addresssize prefixes. [use16] So if we want to use in the 16 bit addressmode 32 bit operands/addresses, then we have to add those prefixes to our opcode. Without those prefixes we can only use 16 bit. [use32] In the 32 bit addressmode we found a diametrical opposite situation, so if we want to use 32 bit operands/addresses, then we have to leave out those prefixes from our opcode and only if we want to use 16 operand/addresses, then we have to add those prefixes to our opcode. Quote: The default segment register is DS for normal registers, SS for SP and CS for IP. The default segment register is DS for normal registers, SS for SP and for BP and CS for IP. ... @deathmood: For using the 16 bit Unrealmode/16 bit FRM we have to switch from the realmode to the protected mode with a GDT with an increased segmentsize of a datasegment deskriptor and with loading a data-segmentregister with the selector of this deskriptor and then switching back to the realmode. While we are in the PM without to have an interrupt vector table(IVT), we have to stop interrupts before we are switching to the PM. After we are back in the RM we can enable the interrupts again. At last we have to open the 20. addressline. Code:
NMI_Port = 70h
START: ; mainroutine
cli
in al, NMI_Port ; stop NMIs
or al, 80h
out NMI_Port, al
in al, 71h
call ESEG ; switch to the 16 bit Unrealmode/16 bit FRM
in al, NMI_Port ; enable NMIs
and al, 7Fh
out NMI_Port, al
in al, 71h
sti
; --------------- GDT and sub-routines----------------
;----------------------------------------------------------------------------
; GDT for the Protected Mode
;----------------------------------------------------------------------------
org START + ((($-START)/64)*64)+64 ; Code-alignment
;----------------------------------------------------------------------------
GDTZEIGER DW ? ; Length of the GDT
DW ? ; Address low -Word:SEGMENTE
DW ? ; Address high-Word:SEGMENTE
DW 0 ; reserved
SEGMENTE DW 0 ; Bits: 0-15 Seg.length(Bit0-15)
DW 0 ; Bits: 0-15 Basis-Address Deskriptor-Table
DB 0 ; Bits:16-23 Basis-Address Deskriptor-Table
DB 0 ; Bits: 0- 7 Access Rigths
DB 0 ; Bits: 0- 3 Seg.length(Bit16-19)/Bit7:1=4KByte/0=1Byte
DB 0 ; Bits:24-31 Basis-Address Deskriptor-Table
;-------------------------------------------- Selektor Segmente
DW 0FFFFh ; Segment length Bits: 0-15
DW 0 ; Address low Bits: 0-15 08h Code (CS)
DB 0 ; Address high Bits:16-23
DB 9Ah ; Access Rights
DB 0 ; Seg.length Bits:16-19 im Bit0-3 /Bit7:1=4KByte/0=1Byte
DB 0 ; Seg.Address Bits:24-31
;--------------------------------------------------- Selektor Segmente
DW 0FFFFh ; Segment length Bits: 0-15
DW 0 ; Address low Bits: 0-15 10h Stack (SS)
DB 0 ; Address high Bits:16-23
DB 92h ; Access Rights
DB 0 ; Seg. length Bits:16-19 im Bit0-3 /Bit7:1=4KByte/0=1Byte
DB 0 ; Seg.Address Bits:24-31
;--------------------------------------------------- Selektor Segmente
DW 0FFFFh ; Segment length Bits: 0-15
DW 0 ; Seg.Address Bits: 0-15 18h (DS,ES,FS,GS)
DB 0 ; Seg.Address Bits:16-23
DB 92h ; Access Rights
DB 0FFh ; Seg. length Bits:16-19 im Bit0-3//Bit7:1=4KByte/0=1Byte
DB 0FFh ; Seg.Address Bits:24-31
;---------------------------------------------------
SEGMENTE_END label WORD
Gdt_Groesse equ (OFFSET SEGMENTE_END - SEGMENTE -1)
;---------------------------------------------------
org START + ((($-START)/32)*32)+32 ; Code-Alignment
;----------------------------------------------------------------------------
ESEG: xor eax, eax
mov ax, cs
mov ds, ax
shl eax, 4 ; EAX = physical-
mov ebx, eax ; Segmentstartaddress
mov WORD PTR[SEGMENTE+0Ah], ax ; in the deskriptor
mov WORD PTR[SEGMENTE+12h], ax ; for CS
ror eax, 10h ; and SS saved into the
mov BYTE PTR[SEGMENTE+0Ch], al ; GDT
mov BYTE PTR[SEGMENTE+14h], al
xor eax, eax ; EAX = null
mov ax, OFFSET SEGMENTE ; 16-Bit-Offset
add ebx, eax ; GDT-Address in the
mov WORD PTR[GDTZEIGER], Gdt_Groesse ; GDT-Deskriptor
mov DWORD PTR[GDTZEIGER+2], ebx
pushf ; save Flags
lgdt FWORD PTR[GDTZEIGER] ; load GDT
mov dx, ss ; save SS
mov eax, cr0 ;
or al, 1 ; switch to the Protected Mode
mov cr0, eax ;
; clear Prefetch-Buffer
DB 0EAh
DW (OFFSET PMODE)
DW 8 ; JMP FAR CS:PMODE
;------------------------------------------------
org START + ((($-START)/32)*32)+32 ; Code-Alignment
;------------------------------------------------
PMODE: mov ax, 10h ; SS-Selektor
mov ss, ax ; 64 KByte
mov ax, 18h
mov ds, ax ; DS,ES,FS,GS-Selektor
; mov es, ax
; mov fs, ax
; mov gs, ax
mov eax, cr0 ; switch back to the realmode
and eax, not 1
mov cr0, eax
; clear Prefetch-Buffer
DB 0EAh
DW (OFFSET RMODE)
AKTSEG DW (SEG RMODE) ; JMP FAR CS:RMODE
;------------------------------------------------
org START + ((($-START)/32)*32)+32
;------------------------------------------------
RMODE: mov ss, dx ; load SS
popf ; load Flags
;----------------------------------------------------------------------------
; switch the 20. adressline on
;----------------------------------------------------------------------------
BIT_FREI: call W_8042 ; Wait for the 8042
jnz BACK
mov al, 0D1h ; write command
out 64h, al
call W_8042 ; ready to recieve data
jnz BACK
mov al, 0DFh ; yes, enable the 20 addressline
out 60h, al
;------------------------------------------------------------
; Wait for the 8042
;------------------------------------------------------------
W_8042: xor cx, cx ; CX=0
STATUS: in al, 64h ; read Status
and al, 2 ; Buffer full?
loopnz STATUS ; loop til no or Timeout
BACK: ret
Dirk |
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18 Feb 2013, 08:34 |
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shutdownall 18 Feb 2013, 18:33
I think then in 32 bit (unreal mode) you have to read with
Code: mov al, [0x000FFFFE] - instead of - mov al, [0xF000FFFE] |
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18 Feb 2013, 18:33 |
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deathmood 19 Feb 2013, 18:12
I have one more question: can I use unreal mode with .com files?
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19 Feb 2013, 18:12 |
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ACP 19 Feb 2013, 22:28
deathmood wrote: I have one more question: can I use unreal mode with .com files? Yes you can. Unreal mode is a "state of segment registers" and limits so basically any DOS structure including PSP and MZ exe header have no direct impact on it whatsoever. The only drawback in case of COM files is that internally DOS loader can not handle files larger than 64kb all together and EXE files can have separate multiple segments for code and data. |
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19 Feb 2013, 22:28 |
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deathmood 20 Feb 2013, 06:57
Ok! Thanks all)))
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20 Feb 2013, 06:57 |
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