textlib.asm。
因为分解为两个文件，互相间的符号引用需要通过EXTERN和GLOBAL这两个关键字。
EXTERN声明符号是其他模块的外部符号，GLOBAL声明本模块的符号是全局符号，其他模块可见。
以下是hexdump3.asm源码，相对于书中的程序，我做了少量改动：
; Executable name : hexdump3
; Version : 1.0
; Created date : 4/15/2009
; Last update : 4/20/2009
; Author : Jeff Duntemann
; Description : A simple hex dump utility demonstrating the use of
; separately assembled code libraries via EXTERN
;
; Build using these commands:
; nasm -f elf -g -F stabs hexdump3.asm
; ld -o hexdump3 hexdump3.o <path>/textlib.o
;
SECTION .bss ; Section containing uninitialized data
BUFFLEN EQU 10
Buff: resb BUFFLEN
SECTION .data ; Section containing initialised data
SECTION .text ; Section containing code
EXTERN ClearLine, DumpChar, PrintLine, DUMPLEN
GLOBAL _start
_start:
nop ; This no-op keeps gdb happy...
nop
xor esi,esi ; Clear total chars counter to 0
; Read a buffer full of text from stdin:
Read:
mov eax,3 ; Specify sys_read call
mov ebx,0 ; Specify File Descriptor 0: Standard Input
mov ecx,Buff ; Pass offset of the buffer to read to
mov edx,BUFFLEN ; Pass number of bytes to read at one pass
int 80h ; Call sys_read to fill the buffer
mov ebp,eax ; Save # of bytes read from file for later
cmp eax,0 ; If eax=0, sys_read reached EOF on stdin
jbe Done ; Jump If Equal (to 0, from compare)
; Set up the registers for the process buffer step:
xor ecx,ecx ; Clear buffer pointer to 0
; Go through the buffer and convert binary values to hex digits:
Scan:
xor eax,eax ; Clear EAX to 0
mov al,byte[Buff+ecx] ; Get a char from the buffer into AL
mov edx,esi ; Copy total counter into EDX
and edx,0000000Fh ; Mask out lowest 4 bits of char counter
call DumpChar ; Call the char poke procedure
; Bump the buffer pointer to the next character and see if buffer's done:
inc ecx ; Increment buffer pointer
inc esi ; Increment total chars processed counter
cmp ecx,ebp ; Compare with # of chars in buffer
jb .modTest ; If we've processed all chars in buffer...
test esi,0000000Fh
jnz .loadBuff
call PrintLine
call ClearLine
.loadBuff:
jmp Read ; If we've done the buffer, go get more
; See if we're at the end of a block of 16 and need to display a line:
.modTest:
test esi,0000000Fh ; Test 4 lowest bits in counter for 0
jnz Scan ; If counter is *not* modulo 16, loop back
call PrintLine ; ...otherwise print the line
call ClearLine ; Clear hex dump line to 0's
jmp Scan ; Continue scanning the buffer
; All done! Let's end this party:
Done:
call PrintLine ; Print the "leftovers" line
mov ecx,DUMPLEN
mov eax,1 ; Code for Exit Syscall
mov ebx,0 ; Return a code of zero
int 80H ; Make kernel call
以下是textlib.asm的源码：
; Executable name : textlib
; Version : 1.0
; Created date : 4/10/2009
; Last update : 9/2/2010
; Author : Jeff Duntemann
; Description : A linkable library of text-oriented procedures and tables
;
; Build using these commands:
; nasm -f elf -g -F stabs textlib.asm
;
SECTION .bss ; Section containing uninitialized data
BUFFLEN EQU 10
Buff resb BUFFLEN
SECTION .data ; Section containing initialised data
; Here we have two parts of a single useful data structure, implementing the
; text line of a hex dump utility. The first part displays 16 bytes in hex
; separated by spaces. Immediately following is a 16-character line delimited
; by vertical bar characters. Because they are adjacent, they can be
; referenced separately or as a single contiguous unit. Remember that if
; DumpLin is to be used separately, you must append an EOL before sending it
; to the Linux console.
GLOBAL DumpLin, HexDigits, BinDigits ;Data items
GLOBAL DumpLength, ASCLength, FullLength, DUMPLEN ;Equate exports
DumpLin: db " 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 "
DUMPLEN EQU $-DumpLin
ASCLin: db "|................|",10
ASCLEN EQU $-ASCLin
FULLLEN EQU $-DumpLin
; The equates shown above must be applied to variables to be exported:
DumpLength: dd 3
ASCLength: dd ASCLEN
FullLength: dd FULLLEN
; The HexDigits table is used to convert numeric values to their hex
; equivalents. Index by nybble without a scale: [HexDigits+eax]
HexDigits: db "0123456789ABCDEF"
; This table allows us to generate text equivalents for binary numbers.
; Index into the table by the nybble using a scale of 4:
; [BinDigits + ecx*4]
BinDigits: db "0000","0001","0010","0011"
db "0100","0101","0110","0111"
db "1000","1001","1010","1011"
db "1100","1101","1110","1111"
; This table is used for ASCII character translation, into the ASCII
; portion of the hex dump line, via XLAT or ordinary memory lookup.
; All printable characters "play through" as themselves. The high 128
; characters are translated to ASCII period (2Eh). The non-printable
; characters in the low 128 are also translated to ASCII period, as is
; char 127.
DotXlat:
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 20h,21h,22h,23h,24h,25h,26h,27h,28h,29h,2Ah,2Bh,2Ch,2Dh,2Eh,2Fh
db 30h,31h,32h,33h,34h,35h,36h,37h,38h,39h,3Ah,3Bh,3Ch,3Dh,3Eh,3Fh
db 40h,41h,42h,43h,44h,45h,46h,47h,48h,49h,4Ah,4Bh,4Ch,4Dh,4Eh,4Fh
db 50h,51h,52h,53h,54h,55h,56h,57h,58h,59h,5Ah,5Bh,5Ch,5Dh,5Eh,5Fh
db 60h,61h,62h,63h,64h,65h,66h,67h,68h,69h,6Ah,6Bh,6Ch,6Dh,6Eh,6Fh
db 70h,71h,72h,73h,74h,75h,76h,77h,78h,79h,7Ah,7Bh,7Ch,7Dh,7Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
db 2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh,2Eh
SECTION .text ; Section containing code
GLOBAL ClearLine, DumpChar, Newlines, PrintLine ;Procedures
;-------------------------------------------------------------------------
; ClearLine: Clear a hex dump line string to 16 0 values
; UPDATED: 4/13/2009
; IN: Nothing
; RETURNS: Nothing
; MODIFIES: Nothing
; CALLS: DumpChar
; DESCRIPTION: The hex dump line string is cleared to binary 0.
ClearLine:
push edx ; Save caller's EDX
mov edx,15 ; We're going to go 16 pokes, counting from 0
.Poke: mov eax,0 ; Tell DumpChar to poke a '0'
call DumpChar ; Insert the '0' into the hex dump string
sub edx,1 ; DEC doesn't affect CF!
jae .Poke ; Loop back if EDX >= 0
pop edx ; Restore caller's EDX
ret ; Go home
;-------------------------------------------------------------------------
; DumpChar: "Poke" a value into the hex dump line string.
; UPDATED: 4/13/2009
; IN: Pass the 8-bit value to be poked in EAX.
; Pass the value's position in the line (0-15) in EDX
; RETURNS: Nothing
; MODIFIES: EAX
; CALLS: Nothing
; DESCRIPTION: The value passed in EAX will be placed in both the hex dump
; portion and in the ASCII portion, at the position passed
; in ECX, represented by a space where it is not a printable
; character.
DumpChar:
push ebx ; Save EBX on the stack so we don't trash
push edi
; First we insert the input char into the ASCII portion of the dump line
mov bl,byte [DotXlat+eax] ; Translate nonprintables to '.'
mov byte [ASCLin+edx+1],bl ; Write to ASCII portion
; Next we insert the hex equivalent of the input char in the hex portion
; of the hex dump line:
mov ebx,eax ; Save a second copy of the input char
lea edi,[edx*2+edx] ; Calc offset into line string (ECX X 3)
; Look up low nybble character and insert it into the string:
and eax,0000000Fh ; Mask out all but the low nybble
mov al,byte [HexDigits+eax] ; Look up the char equivalent of nybble
mov byte [DumpLin+edi+2],al ; Write the char equivalent to line string
; Look up high nybble character and insert it into the string:
and ebx,000000F0h ; Mask out all the but second-lowest nybble
shr ebx,4 ; Shift high 4 bits of char into low 4 bits
mov bl,byte [HexDigits+ebx] ; Look up char equivalent of nybble
mov byte [DumpLin+edi+1],bl ; Write the char equiv. to line string
;Done! Let's go home:
pop edi ; Restore caller's EDI register value
pop ebx ; Restore caller's EBX register value
ret ; Return to caller
;-------------------------------------------------------------------------
; Newlines: Sends between 1-15 newlines to the Linux console
; UPDATED: 4/13/2009
; IN: # of newlines to send, from 1 to 15
; RETURNS: Nothing
; MODIFIES: Nothing
; CALLS: Kernel sys_write
; DESCRIPTION: The number of newline chareacters (0Ah) specified in EDX
; is sent to stdout using using INT 80h sys_write. This
; procedure demonstrates placing constant data in the
; procedure definition itself, rather than in the .data or
; .bss sections.
Newlines:
pushad ; Save all caller's registers
cmp edx,15 ; Make sure caller didn't ask for more than 15
ja .exit ; If so, exit without doing anything
mov ecx,EOLs ; Put address of EOLs table into ECX
mov eax,4 ; Specify sys_write
mov ebx,1 ; Specify stdout
int 80h ; Make the kernel call
.exit popad ; Restore all caller's registers
ret ; Go home!
EOLs db 10,10,10,10,10,10,10,10,10,10,10,10,10,10,10
;-------------------------------------------------------------------------
; PrintLine: Displays the hex dump line string via INT 80h sys_write
; UPDATED: 4/13/2009
; IN: Nothing
; RETURNS: Nothing
; MODIFIES: Nothing
; CALLS: Kernel sys_write
; DESCRIPTION: The hex dump line string is displayed to stdout using
; INT 80h sys_write.
PrintLine:
pushad ; Push all GP registers
mov eax,4 ; Specify sys_write call
mov ebx,1 ; Specify File Descriptor 1: Standard output
mov ecx,DumpLin ; Pass offset of line string
mov edx,FULLLEN ; Pass size of the line string
int 80h ; Make kernel call to display line string
popad ; Pop all GP registers
ret
程序和HexDump2差不多，没有必要再分析。
makefile文件：
hexdump3: hexdump3.o textlib.o
ld -o hexdump3 hexdump3.o textlib.o
textlib.o: textlib.asm
nasm -f elf -g -F stabs textlib.asm
hexdump3.o: hexdump3.asm
nasm -f elf -g -F stabs hexdump3.asm
以下是测试结果：
[root@bogon hexdump3]# make
nasm -f elf -g -F stabs hexdump3.asm
nasm -f elf -g -F stabs textlib.asm
ld -o hexdump3 hexdump3.o textlib.o
[root@bogon hexdump3]# ./hexdump3
123456789012345
31 32 33 34 35 36 37 38 39 30 31 32 33 34 35 0A |123456789012345.|