Spectrum ROM | Routines |
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The address of this routine is found in the table of addresses.
This subroutine gives a 'last value' on the top of the calculator stack that is the result of converting a number given in the form xEm, where m is a positive or negative integer. The subroutine is entered with x at the top of the calculator stack and m in the A register.
The method used is to find the absolute value of m, say p, and to multiply or divide x by 10↑p according to whether m is positive or negative.
To achieve this, p is shifted right until it is zero, and x is multiplied or divided by 10↑(2↑n) for each set bit b(n) of p. Since p is never much more than decimal 39, bits 6 and 7 of p will not normally be set.
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e_to_fp | 11599 | RLCA | Test the sign of m by rotating bit 7 of A into the carry without changing A. | ||||
11600 | RRCA | ||||||
11601 | JR NC,E_SAVE | Jump if m is positive. | |||||
11603 | CPL | Negate m in A without disturbing the carry flag. | |||||
11604 | INC A | ||||||
E_SAVE | 11605 | PUSH AF | Save m in A briefly. | ||||
11606 | LD HL,23698 | This is MEMBOT; a sign flag is now stored in the first byte of mem-0, i.e. 0 for '+' and 1 for '-'. | |||||
11609 | CALL FP_0_1 | ||||||
11612 | RST 40 | The stack holds x. | |||||
11613 | DEFB 164 | stk_ten: x, 10 (decimal) | |||||
11614 | DEFB 56 | end_calc: x, 10 | |||||
11615 | POP AF | Restore m in A. | |||||
E_LOOP | 11616 | SRL A | In the loop, shift out the next bit of m, modifying the carry and zero flags appropriately; jump if carry reset. | ||||
11618 | JR NC,E_TST_END | ||||||
11620 | PUSH AF | Save the rest of m and the flags. | |||||
11621 | RST 40 | The stack holds x' and 10↑(2↑n), where x' is an interim stage in the multiplication of x by 10↑m, and n=0, 1, 2, 3, 4 or 5. | |||||
11622 | DEFB 193 | st_mem_1: (10↑(2↑n) is copied to mem-1) | |||||
11623 | DEFB 224 | get_mem_0: x', 10↑(2↑n), (1/0) | |||||
11624 | DEFB 0 | jump_true to E_DIVSN: x', 10↑(2↑n) | |||||
11625 | DEFB 4 | ||||||
11626 | DEFB 4 | multiply: x'*10↑(2↑n)=x" | |||||
11627 | DEFB 51 | jump to E_FETCH: x'' | |||||
11628 | DEFB 2 | ||||||
E_DIVSN | 11629 | DEFB 5 | division: x/10↑(2↑n)=x'' (x'' is x'*10↑(2↑n) or x'/10↑(2↑n) according as m is '+' or '-') | ||||
E_FETCH | 11630 | DEFB 225 | get_mem_1: x'', 10↑(2↑n) | ||||
11631 | DEFB 56 | end_calc: x'', 10↑(2↑n) | |||||
11632 | POP AF | Restore the rest of m in A, and the flags. | |||||
E_TST_END | 11633 | JR Z,E_END | Jump if m has been reduced to zero. | ||||
11635 | PUSH AF | Save the rest of m in A. | |||||
11636 | RST 40 | x'', 10↑(2↑n) | |||||
11637 | DEFB 49 | duplicate: x'', 10↑(2↑n), 10↑(2↑n) | |||||
11638 | DEFB 4 | multiply: x'', 10↑(2↑(n+1)) | |||||
11639 | DEFB 56 | end_calc: x'', 10↑(2↑(n+1)) | |||||
11640 | POP AF | Restore the rest of m in A. | |||||
11641 | JR E_LOOP | Jump back for all bits of m. | |||||
E_END | 11643 | RST 40 | Use the calculator to delete the final power of 10 reached leaving the 'last value' x*10↑m on the stack. | ||||
11644 | DEFB 2 | delete | |||||
11645 | DEFB 56 | end_calc | |||||
11646 | RET |
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