assembly - アセンブリ x86 で SWORD 値を SDWORD に追加する

Question

.data
 val1 SWORD 8     ;declare signed 16-bit word and initalize with 
 val2 SDWORD 15  ;declare  32-bit signed double-word and initalize with -15
 val3 SDWORD 20   ;initalize SDWORD w/ 20

.code
main PROC

                     ;EAX = -val2 + 7 – val3 + val1

 mov eax,0
 mov eax,val2        ;mov val2 to 32 bit register, eax = 
 neg eax
 add eax,7           ;add 7 register eax =   
 sub eax,val3        ;sub val3 from eax = 
 mov ax,val1
 movsx eax,ax                   ;I dont know if this is the same thing as adding?
 call DumpRegs

Answer 1

.data
 val1 SWORD 8     ; declare signed 16-bit word and initalize with 8
 val2 SDWORD 15   ; declare 32-bit signed double-word and initalize with 15
 val3 SDWORD 20   ; initalize SDWORD w/ 20

.code
main PROC
                     ; eax = -val2 + 7 – val3 + val1

; mov eax, 0         ; unnecessary mov

 mov eax, val2       ; eax = val2
 neg eax             ; eax = -val2
 add eax, 7          ; eax = -val2 + 7
 sub eax, val3       ; eax = -val2 + 7 - val3

; mov ax, val1       ; if you do this, you will lose what you've computed in eax so far
; movsx eax, ax      ; this loads eax with sign-extended ax

; do this instead:
 movsx ebx, val1     ; ebx = val1, correctly sign-extended from 16 to 32 bits
 add eax, ebx        ; eax = -val2 + 7 - val3 + val1

 call DumpRegs

movsxまた、CPUマニュアルのコピーを入手し、なじみのない指示が正確に何をするかについて読むことを忘れないでください。