Glenn's Space

.radix 10
.model tiny
.code

; This program uses almost all of the 64K code segment.  There might
; be rare situations where the last 4 digits of the printout are
; wrong, but this is less likely than in the previous version.
; The last 4 digits should be 1224. (9,304 digits of pi)

; Program assumes there is 64K of memory available for the program,
; without checking.  In the rare case where there isn't 64K
; available, it may crash the computer.
; i.e. don't LOADHIGH this program.

org 100h
start:
  mov    cx,2326*14 ;32564 ; this MUST be a multiple of 14
              ; might be possible to increase
              ; to 2327*14, but that last 4 digits
              ; is much more likely to be corrupted
              ; and/or corrupt the rest of the array
              ; since that ends the scratch area
              ; at FFF1h.
  mov    di,Offset Buffer
  mov    bx,cx
  mov    ax,2000        ; initial value of the array
  mov    bp,10000    ; keeping 10000 constant in extra register
  rep    stosw        ; wipes the buffer used.
              ; offset 0172h through offset FFD9

      ; FFDA-FFFE is stackspace for 18 words.
      ; the workspace shrinks by an additional 14 words
      ; before calling an INT.  This program is smaller
      ; than previous version by 12 bytes, meaning that
      ; stack is larger by 6 words, making it less likely
      ; that the stack will overrun the workspace.

  ; cx = 0000, which is the original value of the remainder.

  ; bx is decremented from 32564 by 14's.  Lesser value would
  ; not have correct values toward the low end of the output.
  ; outer-counter is "i", inner-counter is "j"
  ; because counter is decremented at the end, array is accessed
  ; by subtracting 2 from the counter.

OuterLoop:
  xor    ax,ax
  push    bx
  cwd            ; dx:ax initialized as 0000:0000

InnerLoop:
  push    bx
  mov    si,dx        ; save dx thru the multiply
  mul    bx        ; bx * ax -> dx:ax
  xchg    si,ax        ; get the original dx to multiply it
  push    dx
  mul    bx
  pop    dx
  add    ax,dx
  xchg    di,ax        ; dx:ax * bx -> di:si

  mov    ax,bp        ; ax = 10,000
  add    bx,bx        ; align for 2-byte array elements
  mul    word ptr [bx+offset Buffer-2]
              ; because bx is decremented at the end,
              ; must subract 2 to point at correct item
  add    ax,si
  adc    dx,di        ; dx:ax = dx:ax + di:si
  dec    bx        ; bx = 2*j -1
  xor    di,di
  xchg    di,ax
  xchg    dx,ax
  div    bx        ; dx = 0, ax = high part of prev di:si
              ; dx:ax / bx -> ax, dx=remainder
  xchg    di,ax
  mov    si,dx
  xor    dx,dx
  div    bx        ; dx = 0, ax = low part of prev di:si
  xchg    si,ax
  xchg    dx,ax
  div    bx
  mov    [bx+offset buffer-1],dx    ; because bx was decremented
                  ; above, just subtract 1 to
                  ; access correct element.
  xor    dx,dx
  add    ax,si        ; si:di + ax -> dx:ax
  adc    dx,di
  pop    bx
  dec    bx
  jnz    InnerLoop

  div    bp    ; div by 10,000 - resulting mod has 4 decimal digits

  push    dx    ; save remainder for use in next loop.
  add    ax,cx
  mov    bl,10    ; dividing by 10 yields decimal digits.
  mov    cx,4

Num1:
  cwd        ; "xor dx,dx" not needed, value is always < 7fff
  div    bx    ; divide by 10
  push    dx
  loop    Num1

  mov    cl,4

Num2:
;    pop    dx 
;    add    dl,"0"
;    mov    ah,2
;    int    21H

  pop    ax    ; these 3 lines save 3 bytes compared to
  add    al,"0"    ; the previous 4 lines, but the output
  int    29H    ; can't be redirected to a file

  loop    Num2

  pop    cx    ; restore remainder pushed in DX above
  pop    bx    ; restore outer-loop's value
  sub    bx,+14    ; decrement by 14.

;    jnz    OuterLoop
  jnbe    OuterLoop    ; changed from JNZ in case someone
              ; didn't make BX a multiple of 14.

  ret        ; .COM always has 0000 on the stack.
          ; instruction at offset 0000 is always
          ; an INT 20H, which ends the program.

ProgramEnd:
Buffer=$ ;+1 ;align even without adding a byte

end start
  ; This is offset 016Eh.  Next 32564 words through FFD5h
  ; are used as workspace.