用户界面
我敢说，现代计算系统中 80％的处理不需要用户交互，例如 Linux，OSX 和 Windows 的内核代码。对于那些做的人来说，有两个基本因素是通过键盘（ 指点设备 ）和控制台进行交互。本系列中的这个示例和其他示例都是基于基于文本的控制台（VT100 仿真）和键盘。
就其本身而言，这个例子非常简单，但它是更复杂算法的重要组成部分。
Subrtx.asm
STDIN equ 0
STDOUT equ 1
SYS_READ equ 0
SYS_WRITE equ 1
global gets, strlen, print, atoq
section .text
由于这仅适用于键盘，因此错误概率几乎为零。我想大多数情况下，程序将能够考虑缓冲区大小来规避缓冲区溢出，但由于间接性，这无法保证。
; =============================================================================
; Accept canonical input from operator for a maximum of EDX bytes and replace
; terminating CR with NULL.
; ENTER: RSI = Pointer to input buffer
; EDX = Maximum number of characters
; LEAVE: EAX = Number of characters entered
; R11 = Modified by syscall, all others preserved.
; FLAGS: ZF = Null entry, NZ otherwise.
; _____________________________________________________________________________
gets: push rcx
push rdi
xor eax, eax ; RAX = SYS_READ
mov edi, eax ; RDI = STDIN
syscall
; TODO: Should probably do some error trapping here, especially for
; buffer overrun, but I'll see if it becomes an issue over time.
dec eax ; Bump back to CR
mov byte [rsi+rax], 0 ; Replace it with NULL
pop rdi
pop rcx
ret
首先，这是为了避免编写或手动计算 write(2) 的字符串长度的需要。然后我决定加入一个分隔符，现在它可以用来扫描任何字符（0 - FF）。这打开了自动换行文本的可能性，例如，标签 strlen 有点用词不当，因为人们通常会认为结果将是可见字符的数量。
; =============================================================================
; Determine length, including terminating character EOS. Result may include
; VT100 escape sequences.
; ENTER: RDI = Pointer to ASCII string.
; RCX Bits 31 - 08 = Max chars to scan (1 - 1.67e7)
; 07 - 00 = Terminating character (0 - FF)
; LEAVE: RAX = Pointer to next string (optional).
; FLAGS: ZF = Terminating character found, NZ otherwise (overrun).
; _____________________________________________________________________________
strlen: push rcx ; Preserve registers used by proc so
push rdi ; it's non-destructive except for RAX.
mov al, cl ; Byte to scan for in AL.
shr ecx, 8 ; Shift max count into bits 23 - 00
; NOTE: Probably should check direction flag here, but I always set and
; reset DF in the process that is using it.
repnz scasb ; Scan for AL or until ECX = 0
mov rax, rdi ; Return pointer to EOS + 1
pop rdi ; Original pointer for proglogue
jz $ + 5 ; ZF indicates EOS was found
mov rax, rdi ; RAX = RDI, NULL string
pop rcx
ret
此过程的目的是简化调用过程中的循环设计。
; =============================================================================
; Display an ASCIIZ string on console that may have embedded VT100 sequences.
; ENTER: RDI = Points to string
; LEAVE: RAX = Number of characters displayed, including EOS
; = Error code if SF
; RDI = Points to byte after EOS.
; R11 = Modified by syscall all others preserved
; FLAGS: ZF = Terminating NULL was not found. NZ otherwise
; SF = RAX is negated syscall error code.
;______________________________________________________________________________
print: push rsi
push rdx
push rcx
mov ecx, -1 << 8 ; Scan for NULL
call strlen
push rax ; Preserve point to next string
sub rax, rdi ; EAX = End pntr - Start pntr
jz .done
; size_t = write (int STDOUT, char *, size_t length)
mov edx, eax ; RDX = length
mov rsi, rdi ; RSI = Pointer
mov eax, SYS_WRITE
mov edi, eax ; RDI = STDOUT
syscall
or rax, rax ; Sets SF if syscall error
; NOTE: This procedure is intended for console, but in the event STDOUT is
; redirected by some means, EAX may return error code from syscall.
.done: pop rdi ; Retrieve pointer to next string.
pop rcx
pop rdx
pop rsi
ret
最后是一个如何使用这些功能的例子。
Generic.asm
global _start
extern print, gets, atoq
SYS_EXIT equ 60
ESC equ 27
BSize equ 96
section .rodata
Prompt: db ESC, '[2J' ; VT100 clear screen
db ESC, '[4;11H' ; " Position cursor to line 4 column 11
db 'ASCII -> INT64 (binary, octal, hexidecimal, decimal), '
db 'Packed & Unpacked BCD and floating point conversions'
db 10, 10, 0, 9, 9, 9, '=> ', 0
db 10, 9, 'Bye'
db ESC, '[0m' ; VT100 Reset console
db 10, 10, 0
section .text
_start: pop rdi
mov rsi, rsp
and rsp, byte 0xf0 ; Align stack on 16 byte boundary.
call main
mov rdi, rax ; Copy return code into ARG0
mov eax, SYS_EXIT
syscall
; int main ( int argc, char *args[] )
main: enter BSize, 0 ; Input buffer on stack
mov edi, Prompt
call print
lea rsi, [rbp-BSize] ; Establish pointer to input buffer
mov edx, BSize ; Max size for read(2)
.Next: push rdi ; Save pointer to "=> "
call print
call gets
jz .done
call atoq ; Convert string pointed to by RSI
pop rdi ; Restore pointer to prompt
jmp .Next
.done: call print ; RDI already points to "Bye"
xor eax, eax
leave
ret
Makefile 文件
OBJECTS = Subrtx.o Generic.o
Generic : $(OBJECTS)
ld -oGeneric $(OBJECTS)
readelf -WS Generic
Generic.o : Generic.asm
nasm -g -felf64 Generic.asm
Subrtx.o : Subrtx.asm
nasm -g -felf64 Subrtx.asm
clean:
rm -f $(OBJECTS) Generic