刚开始接触u-boot中的start.S文件时,对汇编程序中的.word很是不解,于是上网上查找资料,整理如下,在此感谢论坛中无私解答的高手们
$ O" j# \+ t5 xstart.S汇编程序中有这么一句: 7 D) }- E1 Q6 h2 m( s
. B7 R V' r- x
ldr pc, _undefined_instruction
+ b% t3 O6 @& D& n" W' S
- ^9 _2 F+ r4 @! E4 \_undefined_instruction: .word undefined_instruction
- I( G$ z0 E5 }% n. m
; @2 o) M% u M. a8 \5 i5 A.word的用法格式
. Q4 o- C6 k& T6 S7 rSyntax: .word expressions # y& D J! u$ O, A: K, C. o
在当前位置放一个word型的值,这个值就是expression % x8 H: a# k$ \ t+ O
, ^" n' u4 |% ~, G2 R# i
在论坛中wheelz的举例解答: 3 `; _) G% s9 t) \0 s
6 p( U" [7 m, K& S$ l: p
ldr r1, _rWTCON
6 U& I; X5 J) @/ Z% @+ h& N" X8 P& g1 M, k
_rWTCON: - P( `9 b$ z3 s# F* a: p6 R0 j- N
.word 0x15300000
7 h2 `, m% b* b. @& Q' {" q+ n+ Q
是把地址_rWTCON上的内容放到r1,而地址_rWTCON上的内容是0x15300000。实际上就是把r1设置为0x15300000 4 I3 j" _, `/ A9 a4 b, ^
8 F' D W5 y, s
' C# I. \& |: z.word和0x1530 0000有什么关系?.word就是把expression上的值取出来? expression代表地址?
7 S% ]# e! I( P# D" {# E! h! w.word expression就是在当前位置放一个word型的值,这个值就是expression
/ y w! m7 \, E+ F1 W6 y- E举例来说,
$ y! l# [/ m+ h4 ^% d0 |/ o. \. ^, w_rWTCON: 3 n- f: l* @ v# D, ]
.word 0x15300000
, P( y" k3 L8 {# l. n; t就是在当前地址,即_rWTCON处放一个值0x15300000 ; G/ _0 h2 Q/ V3 D3 ?
3 F+ p1 u# u3 g3 m翻译成intel的汇编语句就是: % h; D- ]: m8 o' p, h
_rWTCON dw 0x15300000 6 l, W/ x8 o8 ~" x5 M( Y( i
5 m2 b$ g" P' [" j$ u! R6 A% k- V" E' g6 f
: ]1 |$ s+ m+ C, M6 n3 r& j' b( y原汇编程序:(start.S) 1 t! X8 q2 H- i; H0 G' G1 M, ~
0 {$ o: G7 Y# ?5 B4 S7 K8 Q
.globl _start 1 a/ y& d l# i7 E9 n: u
_start: b reset H& I8 D" i% i4 v
ldr pc, _undefined_instruction
, r7 e. Y3 D5 ?0 I, x% L! Sldr pc, _software_interrupt 0 I" |# `/ H2 p4 l( S: H
ldr pc, _prefetch_abort
3 ]. C' A; W; @; w; J: Aldr pc, _data_abort 4 x% A7 i9 I! ~) R
ldr pc, _not_used 9 d, @/ r4 L/ o' E% e7 g/ F
ldr pc, _irq n _! z: j$ f+ U5 N! L- I$ |
ldr pc, _fiq
+ p# P6 s- M5 G; V' y
' a- N1 M1 T3 L( p) k* j4 n_undefined_instruction: .word undefined_instruction
/ l1 P# ^" Q u& Q_software_interrupt: .word software_interrupt
$ P' f% C% l) h/ A/ f_prefetch_abort: .word prefetch_abort 3 C5 p; R# P# l4 O' w' D
_data_abort: .word data_abort : a+ \' c" o; B L) q4 a# @
_not_used: .word not_used
) D: p4 P" w9 Z8 Z_irq: .word irq
4 Y" Z- u- p7 d S_fiq: .word fiq 7 z6 K$ f8 q/ w* S! I
( J" V$ E8 i! D0 L
9 c& V# V8 g$ ^_undefined_instruction是汇编中的标号,代表一个地址,根据反汇编: ) \5 H, I- t! c3 M8 R% R4 [
00000000 [0xea000012] b 0x50 % d) c2 f! X# y
00000004 [0xe59ff014] ldr pc,0x00000020 ; = #0x33f80140 ) `0 x5 w# G4 P4 x
00000008 [0xe59ff014] ldr pc,0x00000024 ; = #0x33f801a0 . k; U9 N8 Y" W' L* [1 o
0000000c [0xe59ff014] ldr pc,0x00000028 ; = #0x33f80200
. X' N1 D7 M* @7 `/ C8 ~00000010 [0xe59ff014] ldr pc,0x0000002c ; = #0x33f80260
" H0 s, ?# u+ z, }00000014 [0xe59ff014] ldr pc,0x00000030 ; = #0x33f802c0
$ m- w" m" L: q* U00000018 [0xe59ff014] ldr pc,0x00000034 ; = #0x33f80320
) o! w: v3 E1 O. q5 B& d7 G0000001c [0xe59ff014] ldr pc,0x00000038 ; = #0x33f80380
- |3 Z8 U4 a8 s; z+ [& O; Z00000020 [0x33f80140] mvnccs r0,#0x10 ; ? rn = 0x8 * n% m! k# r# n# f
00000024 [0x33f801a0] mvnccs r0,#0x28 ; ? rn = 0x8
2 K# ?' O3 B; i( W' m7 X00000028 [0x33f80200] mvnccs r0,#0, 4 ; ? rn = 0x8
+ n. R6 p' a p1 F4 X& _6 F. T0000002c [0x33f80260] mvnccs r0,#6 ; ? rn = 0x8
" D0 a& ]. b5 o, l2 A; _ f00000030 [0x33f802c0] mvnccs r0,#0xc ; ? rn = 0x8
6 Q) n6 M0 w. [, x00000034 [0x33f80320] mvnccs r0,#0x80000000 ; ? rn = 0x8 $ P/ O1 S1 {/ @' U$ Z
00000038 [0x33f80380] mvnccs r0,#2 ; ? rn = 0x8
% |# F0 _( W# u* Z* U. H3 U2 a0000003c [0xdeadbeef] cdple p14,0xa,c11,c13,c15,7
' Y6 q; K2 p& H0 R. B" Q+ s1 @00000040 [0x33f80000] mvnccs r0,#0 ; ? rn = 0x8
9 J' v% C* r4 B( o( i4 z00000044 [0x33f80000] mvnccs r0,#0 ; ? rn = 0x8
7 u1 i5 ^6 T% ?00000048 [0x33f96650] mvnccs r6,#0x5000000 ; ? rn = 0x9
; A {4 D% ~- J4 Q) y8 r0000004c [0x33f9ab80] mvnccs r10,#0x20000 ; ? rn = 0x9
# w" Q* k8 Z% [: w00000050 [0xe10f0000] mrs r0,cpsr
- C% ]0 D' B( f7 k7 S9 x4 g00000054 [0xe3c0001f] bic r0,r0,#0x1f
* n- N- v3 \2 M( X j" }9 _00000058 [0xe38000d3] orr r0,r0,#0xd3
6 G9 c4 ?, l9 i- }0000005c [0xe129f000] msr cpsr_cf,r0
, ]9 R% J" g. H- ~; A * `+ ^( a* s" h4 l2 {
由此可以看出,_undefined_instruction标号代表的地址时0x00000020 在地址0x00000020处分配一个字内存空间,存放undefined_instruction这个标号的地址,当出现未定义指令异常时,将undefined_instruction地址赋给PC寄存器。
) c8 C+ ]2 e' j3 l/ v; d
2 u8 U' E" ~' _. a0 l& \+ ^' o$ |1 i: q8 y" M8 R0 o
4 G c# ?! W1 v5 c0 B p. S. l
|