我通过 UART 收到了一个字符，需要验证它是否是数字字符。

通常我会这样做

if (char >= '0' && char <= '9') { /* VALID */ }

然而，我必须这样做组装中.

我还没有找到任何比较指令（所以我假设没有）。

我怎样才能做到这一点？

    mov A, SBUF    ; load the number

    ; -- pseudocode --
    cmp A, #'0'    ; In AVR, I'd do it this way
    brlt fail      ; but I'm new to 8051
    cmp A, #'9'
    brge fail
    ; -- pseudocode --

    ; number is good

fail:

编辑：好的，这就是我现在所拥有的，但它不起作用

;======================================================
; check if r1 is number char -> r0 = 1 or 0
;------------------------------------------------------
fn_isnum:
    PUSH acc
    PUSH 1

    MOV r1,A

    SUBB A,#'0'
    JC isnum_bad

    MOV A,r1

    SUBB A,#'9'+1
    JC isnum_bad

    POP 1
    POP acc

    MOV r0,#1
    RET 

isnum_bad:

    POP 1
    POP acc

    MOV r0,#0
    RET
;======================================================

Using 这项技术 https://stackoverflow.com/q/17095324/995714, if (a >= '0' && a <= '9')可以转化为

if ((unsigned char)(a - '0') <= ('9'-'0'))

这可以节省你的比较和跳转。现在只需一次比较就足够了

结果可能是这样的

    SUBB A, #'0'  ;  A = a - '0'
    CLR  C
    MOV  R1, A    ; R1 = a - '0'
    MOV  A, #9    ;  A = '9' - '0'
    SUBB A, R1    ;  C = 1 if ('9' - '0') < (a - '0')
    JC   bad:     ; jump when C != 0, i.e. !((a - '0') <= ('9' - '0'))
valid:
    ; do something
bad:

大多数现代编译器都知道如何优化此范围检查。我找不到 8051 在线编译器，也没有离线编译器，但 AVR 足够接近，可以提供演示。 AVR gcc 给出相同的输出原始状态和变换后的状态 https://gcc.godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(j:1,lang:c%2B%2B,source:'char+getMask(char+a)%0A%7B%0A++++return+(unsigned+char)(a+-+!'0!')+%3C%3D+(!'9!'-!'0!')%3B%0A%7D%0A%0Achar+getMask2(char+a)%0A%7B%0A++++return+a+%3E%3D+!'0!'+%26%26+a+%3C%3D+!'9!'%3B%0A%7D%0A'),l:'5',n:'0',o:'C%2B%2B+source+%231',t:'0')),k:31.17831909738605,l:'4',m:100,n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:avrg464,filters:(b:'0',binary:'1',commentOnly:'0',demangle:'0',directives:'0',execute:'1',intel:'0',trim:'1'),lang:c%2B%2B,libs:!(),options:'-O3',source:1),l:'5',n:'0',o:'AVR+gcc+4.6.4+(Editor+%231,+Compiler+%231)+C%2B%2B',t:'0')),header:(),k:35.48834756928063,l:'4',m:100,n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:clang_trunk,filters:(b:'0',binary:'1',commentOnly:'0',demangle:'0',directives:'0',execute:'1',intel:'0',trim:'1'),lang:c%2B%2B,libs:!(),options:'-O3',source:1),l:'5',n:'0',o:'x86-64+clang+(trunk)+(Editor+%231,+Compiler+%232)+C%2B%2B',t:'0')),k:33.33333333333333,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4

    mov r25,r24          ; input in r24
    subi r25,lo8(-(-48)) ; r25 = input - '0'
    ldi r24,lo8(1)       ; r24 = 1
    cpi r25,lo8(10)      ; if r25 < 10
    brlo .L2             ; jump to .L2
    ldi r24,lo8(0)       ; r24 = 0
.L2:
    ret                  ; return value in r24

Update:

8051 的输出示例 https://gcc.godbolt.org/#g:!((g:!((g:!((h:codeEditor,i:(fontScale:14,j:1,lang:___c,selection:(endColumn:1,endLineNumber:10,positionColumn:1,positionLineNumber:10,selectionStartColumn:1,selectionStartLineNumber:10,startColumn:1,startLineNumber:10),source:'char+getMask(char+a)%0A%7B%0A++++return+(unsigned+char)(a+-+!'0!')+%3C%3D+(!'9!'-!'0!')%3B%0A%7D%0A%0Achar+getMask2(char+a)%0A%7B%0A++++return+a+%3E%3D+!'0!'+%26%26+a+%3C%3D+!'9!'%3B%0A%7D%0A'),l:'5',n:'0',o:'C+source+%231',t:'0')),k:32.13585891574134,l:'4',m:100,n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:sdcc400,filters:(b:'0',binary:'1',commentOnly:'0',demangle:'0',directives:'0',execute:'1',intel:'0',libraryCode:'1',trim:'1'),fontScale:14,j:1,lang:___c,libs:!(),options:'--opt-code-speed',selection:(endColumn:23,endLineNumber:17,positionColumn:23,positionLineNumber:17,selectionStartColumn:23,selectionStartLineNumber:17,startColumn:23,startLineNumber:17),source:1),l:'5',n:'0',o:'SDCC+4.0.0+(Editor+%231,+Compiler+%231)+C',t:'0')),header:(),k:67.86414108425865,l:'4',m:100,n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4来自SDCC

        mov     r7,dpl
        cjne    r7,#0x30,00110$
00110$:
        jc      00103$
        mov     a,r7
        add     a,#0xff - 0x39
        jnc     00104$
00103$:
        mov     r7,#0x00
        sjmp    00105$
00104$:
        mov     r7,#0x01
00105$:
        mov     dpl,r7
        ret