2字节有符号整数比较汇编算法问题排查求助

初始实现版本 #

我正在实现2字节有符号整数的小于比较功能，多次重写算法后仍无法覆盖所有测试用例。以下是最新版本代码：

MATH_lt_int: ; a < b, a: MATH_INT_INPUT_1, b: MATH_INT_INPUT_2
  jsr MATH_clear_int_output ; I use too many subroutines, sorry.
  lda MATH_INT_INPUT_1 + 1
  eor MATH_INT_INPUT_2 + 1
  bmi @different_sign ; eor our high bytes and check
                      ; the neg flag to see if they differ
  lda MATH_INT_INPUT_1 + 1 ; if one is positive here, they both must be
  bpl @both_positive
@both_negative: ; we have to do an inverted comparison here
                ; because if a < b then -b < -a
  sec
  lda MATH_INT_INPUT_1 + 1 ; check high byte first
  cmp MATH_INT_INPUT_2 + 1
  bcc @not_less_than ; inverted because negative
  sec
  lda MATH_INT_INPUT_1 ; low byte next
  cmp MATH_INT_INPUT_2
  ; the problem seems to be somewhere around here
  bcc @not_less_than ; inverted because negative
  beq @not_less_than
  jmp @less_than ; inverted because negative
@both_positive: ; we get to do a straight-up comparison here
  sec
  lda MATH_INT_INPUT_1 + 1 ; check high byte first
  cmp MATH_INT_INPUT_2 + 1
  bcc @less_than
  sec
  lda MATH_INT_INPUT_1 ; low byte next
  cmp MATH_INT_INPUT_2
  bcc @less_than
  jmp @not_less_than
@different_sign: ; if they have different signs, the negative one is smaller
  lda MATH_INT_INPUT_1 + 1
  bmi @less_than ; are we the negative one? Than we're @less_than
  jmp @not_less_than
@less_than:
  lda #1
  jmp @return
@not_less_than:
  lda #0
@return:
  sta MATH_INT_OUTPUT
  ; return
  rts

测试结果 #

50<50==0   => 0
50<60==1   => 1
60<50==0   => 0
-50<-60==0 => 1
-60<-50==1 => 1

当前问题：当a>b且两者均为负数时，应返回0，但实际返回1，问题疑似出在@both_negative段的第二条bcc @not_less_than语句附近。

更新后的简化版本 #

感谢@penguin359的建议，我将代码简化为如下版本，但仍无法正确处理负数比较：

MATH_lt_int: ; a < b, a: MATH_INT_INPUT_1, b: MATH_INT_INPUT_2
  jsr MATH_clear_int_output ; I use too many subroutines, sorry.
  sec
  lda MATH_INT_INPUT_1 + 1 ; check high byte first
  cmp MATH_INT_INPUT_2 + 1
  bmi @less_than
  sec
  lda MATH_INT_INPUT_1 ; low byte next
  cmp MATH_INT_INPUT_2
  bmi @less_than
  jmp @not_less_than
@less_than:
  lda #1
  jmp @return
@not_less_than:
  lda #0
@return:
  sta MATH_INT_OUTPUT
  ; return
  rts

测试结果 #

50<50==0   => 0
50<60==1   => 1
60<50==0   => 0
-50<-60==0 => 0
-60<-50==1 => 0

内容的提问来源于stack exchange，提问作者JShoe