Routines 
Prev: 30CA  Up: Map 
The address of this routine is found in the table of addresses. It is called via the calculator literal +05 by the routines at BEEP, DRAW, CD_PRMS1, DEC_TO_FP, e_to_fp, n_mod_m, tan, atn, asn and to_power. It is also called indirectly via fp_calc_2.
This subroutine first prepares the divisor by calling PREP_M_D, reporting arithmetic overflow if it is zero; then it prepares the dividend again calling PREP_M_D, returning if it is zero. Next it fetches the two numbers from the calculator stack and divides their mantissa by means of the usual restoring division, trial subtracting the divisor from the dividend and restoring if there is carry, otherwise adding 1 to the quotient. The maximum precision is obtained for a 4byte division, and after subtracting the exponents the subroutine exits by joining the later part of multiply.


division  31AF  CALL RE_ST_TWO  Use full floatingpoint forms.  
31B2  EX DE,HL  Exchange the pointers.  
31B3  XOR A  A is set to 0, so that the sign of the first number will go into A.  
31B4  CALL PREP_M_D  Prepare the divisor and give the report for arithmetic overflow if it is zero.  
31B7  JR C,REPORT_6  
31B9  EX DE,HL  Exchange the pointers.  
31BA  CALL PREP_M_D  Prepare the dividend and return if it is zero (result already zero).  
31BD  RET C  
31BE  EXX  Exchange the pointers.  
31BF  PUSH HL  Save the next literal address.  
31C0  EXX  Exchange the registers.  
31C1  PUSH DE  Save pointer to divisor.  
31C2  PUSH HL  Save pointer to dividend.  
31C3  CALL FETCH_TWO  Get the two numbers from the stack.  
31C6  EXX  Exchange the registers.  
31C7  PUSH HL  Save M1 and N1 (the exponent bytes) on the machine stack.  
31C8  LD H,B  Copy the four bytes of the dividend from registers B'C'CB (i.e. M2, M3, M4 and M5; see FETCH_TWO) to the registers H'L'HL.  
31C9  LD L,C  
31CA  EXX  
31CB  LD H,C  
31CC  LD L,B  
31CD  XOR A  Clear A and reset the carry flag.  
31CE  LD B,$DF  B will count upwards from 33 to 1 (+DF to +FF), looping on minus and will jump again on zero for extra precision.  
31D0  JR DIV_START  Jump forward into the division loop for the first trial subtraction.  
Now enter the division loop.


DIV_LOOP  31D2  RLA  Shift the result left into B'C'CA, shifting out the bits already there, picking up 1 from the carry whenever it is set, and rotating left each byte with carry to achieve the 32bit shift.  
31D3  RL C  
31D5  EXX  
31D6  RL C  
31D8  RL B  
31DA  EXX  
DIV_34TH  31DB  ADD HL,HL  Move what remains of the dividend left in H'L'HL before the next trial subtraction; if a bit drops into the carry, force no restore and a bit for the quotient, thus retrieving the lost bit and allowing a full 32bit divisor.  
31DC  EXX  
31DD  ADC HL,HL  
31DF  EXX  
31E0  JR C,SUBN_ONLY  
DIV_START  31E2  SBC HL,DE  Trial subtract divisor in D'E'DE from rest of dividend in H'L'HL; there is no initial carry (see previous step).  
31E4  EXX  
31E5  SBC HL,DE  
31E7  EXX  
31E8  JR NC,NO_RSTORE  Jump forward if there is no carry.  
31EA  ADD HL,DE  Otherwise restore, i.e. add back the divisor. Then clear the carry so that there will be no bit for the quotient (the divisor 'did not go').  
31EB  EXX  
31EC  ADC HL,DE  
31EE  EXX  
31EF  AND A  
31F0  JR COUNT_ONE  Jump forward to the counter.  
SUBN_ONLY  31F2  AND A  Just subtract with no restore and go on to set the carry flag because the lost bit of the dividend is to be retrieved and used for the quotient.  
31F3  SBC HL,DE  
31F5  EXX  
31F6  SBC HL,DE  
31F8  EXX  
NO_RSTORE  31F9  SCF  One for the quotient in B'C'CA.  
COUNT_ONE  31FA  INC B  Step the loop count up by one.  
31FB  JP M,DIV_LOOP  Loop 32 times for all bits.  
31FE  PUSH AF  Save any 33rd bit for extra precision (the present carry).  
31FF  JR Z,DIV_START  Trial subtract yet again for any 34th bit; the 'PUSH AF' above saves this bit too.  
Note: this jump is made to the wrong place. No 34th bit will ever be obtained without first shifting the dividend. Hence important results like 1/10 and 1/1000 are not rounded up as they should be. Rounding up never occurs when it depends on the 34th bit. The jump should have been to DIV_34TH above, i.e. byte +3200 in the ROM should read +DA instead of +E1.


3201  LD E,A  Now move the four bytes that form the mantissa of the result from B'C'CA to D'E'DE.  
3202  LD D,C  
3203  EXX  
3204  LD E,C  
3205  LD D,B  
3206  POP AF  Then put the 34th and 33rd bits into B' to be picked up on normalisation.  
3207  RR B  
3209  POP AF  
320A  RR B  
320C  EXX  
320D  POP BC  Restore the exponent bytes M1 and N1.  
320E  POP HL  Restore the pointer to the result.  
320F  LD A,B  Get the difference between the two exponent bytes into A and set the carry flag if required.  
3210  SUB C  
3211  JP DIVN_EXPT  Exit via DIVN_EXPT. 
Prev: 30CA  Up: Map 