random.asm - Computer Organization

;random.asm            Designed and written by Wilson Seto
      page   60,130
cseg      segment
      assume cs:cseg, ds:cseg
      assume es:cseg, ss:cseg
      org    100h
start:    jmp    short check
;---------TSR routine-----------------------------------------------------------
constant  dw     8405h             ;multiplier value
seed1     dw     ?
seed2     dw     ?             ;random number seeds
int_62h   proc   far
      or     ax,ax             ;range value <> 0?
      jz     abort             ;if not then abort
      push   bx
      push   cx
      push   dx
      push   ds
      push   ax            ;save the range value
      push   cs
      pop    ds            ;ds = cs
      mov    ax,seed1
      mov    bx,seed2          ;load seeds
      mov    cx,ax             ;save seed
      mul    constant          ;(dx,ax) = ax * constant
      shl    cx,1
      shl    cx,1
      shl    cx,1
      add    ch,cl
      add    dx,cx
      add    dx,bx
      shl    bx,1              ;begin scramble algorithm
      shl    bx,1
      add    dx,bx
      add    dh,bl
      mov    cl,5
      shl    bx,cl
      add    ax,1
      adc    dx,0
      mov    seed1,ax
      mov    seed2,dx          ;save results as the new seeds
      pop    bx            ;get back range value
      xor    ax,ax             ;clear register
      xchg   ax,dx             ;adjust ordering
      div    bx            ;ax = trunc((dx,ax) / bx), dx = (r)
      xchg   ax,dx             ;return remainder as the random number
      pop    ds
      pop    dx
      pop    cx
      pop    bx
abort:    iret                 ;return to caller
int_62h   endp
;-------------------------------------------------------------------------------

;---------Initialization routine------------------------------------------------
check:    mov    ax,3000h          ;get DOS version
      int    21h
      cmp    al,3              ;DOS version 3.0 or higher?
      jae    chk_vec
      mov    dx,offset wrong_dos   ;incorrect DOS version, inform user
quit:     mov    ah,9
      int    21h
      int    20h               ;back to DOS
chk_vec:  mov  ax,3562h        ;get interrupt vector for int 62h - es:bx
      int    21h
    mov  di,bx           ;es:di points to int 62h
    mov  si,offset int_62h     ;point to our copy of int 62h
      mov    cx,16             ;# of bytes to compare
      cld                  ;forward string movement
      repe   cmpsb             ;compare memory regions
      jnz    install           ;if not identical then not installed
      mov    dx,offset loaded      ;already loaded, inform user
      jmp    short quit
install:  mov    ah,2ch            ;get current time
      int    21h
      mov    seed1,cx
      mov    seed2,dx          ;save as initial seeds
      mov    dx,offset ready
      mov    ah,9              ;print ready message
      int    21h
    mov  dx,offset int_62h     ;point to routine to install - ds:dx
    mov  ax,2562h        ;change interrupt 62h vectors
      int    21h
      mov    dx,offset check       ;end of resident code
      int    27h               ;TSR
;-------------------------------------------------------------------------------




;---------Data area-------------------------------------------------------------
wrong_dos db     13,10,10,'Requires DOS V3.0 or higher!',13,10,10,7,7,36
loaded      db     13,10,10,'Random number generator is already loaded.'
          db     13,10,10,7,7,36
ready       db     13,10,10,'Random number generator V1.1 loaded.',13,10,10,36
;-------------------------------------------------------------------------------
cseg      ends
      end    start