Difference between two assembly code

Question

I need help understanding a code in assembly language, in terms of declaring variables as local or global variables.

What is the difference between this code

;File: fig0647.pep
;Computer Systems, Fourth edition
;Figure 6.47
;
     BR      main        
data:    .EQUATE 0           ;struct field #2d
next:    .EQUATE 2           ;struct field #2h
;
;******* main ()
first:   .EQUATE 4           ;local variable #2h
p:       .EQUATE 2           ;local variable #2h
value:   .EQUATE 0           ;local variable #2d
main:    SUBSP   6,i         ;allocate #first #p #value
     LDA     0,i         ;first = 0
     STA     first,s     
     DECI    value,s     ;cin >> value
while:   LDA     value,s     ;while (value != -9999)
     CPA     -9999,i     
     BREQ    endWh       
     LDA     first,s     ;   p = first
     STA     p,s         
     LDA     4,i         ;   first = new node
     CALL    new         ;   allocate #data #next
     STX     first,s     
     LDA     value,s     ;   first->data = value
     LDX     data,i      
     STA     first,sxf   
     LDA     p,s         ;   first->next = p
     LDX     next,i      
     STA     first,sxf   
     DECI    value,s     ;   cin >> value
     BR      while       
     endWh:   LDA     first,s     ;for (p = first
     STA     p,s         
     for:     LDA     p,s         ;   p != 0
     CPA     0,i         
     BREQ    endFor      
     LDX     data,i      ;   cout << p->data
     DECO    p,sxf       
     CHARO   ' ',i       ;      << ' '
     LDX     next,i      ;   p = p->next)
     LDA     p,sxf       
     STA     p,s         
     BR      for         
     endFor:  ADDSP   6,i         ;deallocate #value #p #first
     STOP                
     ;
     ;******* operator new
     ;        Precondition: A contains number of bytes
     ;        Postcondition: X contains pointer to bytes
     new:     LDX     hpPtr,d     ;returned pointer
     ADDA    hpPtr,d     ;allocate from heap
     STA     hpPtr,d     ;update hpPtr
     RET0                
     hpPtr:   .ADDRSS heap        ;address of next free byte
     heap:    .BLOCK  1           ;first byte in the heap
     .END

And this code

         BR      main
         data:    .EQUATE 0           ;struct field #2d
         next:    .EQUATE 2           ;struct field #2h
         ;
         ;.........main()
         first:   .BLOCK  2           ;global variable #2h
         p:       .BLOCK  2           ;global variable #2h     
         value:   .BLOCK  2           ;global variable #2d
         main:    LDA     0,i         ;first = 0
         STA     first,s
         DECI    value,s     ;cin >> value
         while:   LDA     value,s     ;while (value != -9999)
         CPA     -9999,i     
         BREQ    endWh
         LDA     first,s     ;    p = first
         STA     p,s
         LDA     4,i         ;    first = new node
         CALL    new         ;    allocate #data #next
         STX     first,s
         LDA     value,s     ;    first->data = value     
         LDX     data,i
         STA     first,sxf
         LDA     p,s         ;    first->next = p
         LDX     next,i

         STA     first,sxf
         DECI    value,s     ;    cin >> value
         BR      while
         endWh:   LDA     first,s     ;for (p=first)
         STA     p,s
         for:     LDA     p,s         ;    p != 0
         CPA     0,i
         BREQ    endFor
         LDX     data,i      ;    couunt << p->data
         DECO    p,sxf
         CHARO   ' ',s       ;        <<  ' '
         LDX     next,s      ;    p = p->next)
         LDA     p,sxf
         STA     p,s
         BR      for
         endFor:  STOP
         ;
         ;....... operator new
         ;        Precondition: A contains number of bytes
         ;        Postcondition: X contains pointer to byte
         new:     LDX     hpPtr,d     ;returned pointer
         ADDA    hpPtr,d     ;allocate from heap
         STA     hpPtr,d     ;update hpPtr
         RET0
         hpPtr:   .ADDRSS heap        ;address of next free byte
         heap:    .BLOCK  1           ;first byte in the heap
         .END

Any kind of help would be appreciated.

Tommylee2k · Accepted Answer · 2017-05-08T13:15:47.207

Variant 1:
Adds 6 bytes to some stack pointer at startup (which reserves 6 bytes for the variables), "first","p","value" just are aliases for the assembler ( to know which variable should be stored, the same thing that #define first 2 would do in C ), and there is no code generated for these .Equate

BR main                      ; <-- this BR is unneeded, there's no code from here to main
...
first:   .EQUATE 4           ;local variable #2h
p:       .EQUATE 2           ;local variable #2h
value:   .EQUATE 0           ;local variable #2d
main:    SUBSP   6,i         ;allocate #first #p #value

Variant2:
is adding the variables directly into the code, "first", "p", "value" are labels. This code is longer (since the variables are somewhat part of the code segment). This only works, if the program is running in memory (since the variables must be changeable) and wouldn't work e.g. in a ROM

BR      main            ; this BR makes sense, there's code inbetween this and main
first:   .BLOCK  2           ;<-- here e.g. a "00 00" appears INSIDE the code
p:       .BLOCK  2           ;global variable #2h     
value:   .BLOCK  2           ;global variable #2d
main:    LDA     0,i

Also note the BR main:
v1: Since the space for the variables is inside the code, you have to jump over them to make sure they're not "executed".
v2: the "BR main" is not needed, since there is no code, but only alias defines

Difference between two assembly code

1 Answers1