# Sqrt.asm

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## Latest revision as of 09:26, 4 December 2010

## Contents |

# Functions Provided

Entry point | Function provided | Notes |
---|---|---|

SQRT | Calculate the square root of a fixed-point number | Value in register, fractional point (Qpt) in ROM. |

SQRT.1 | Calculate the square root of an integer | Value in register, fractional point implied to be 0. |

SQRT.2 | Calculate the square root of a fixed-point number | Value and fractional point both in registers |

See source code below for calling convention.

# Examples

(todo... please contribute!)

# Notes

Unlike the square-root code in the EXEC, this code works over the entire range $0000 to $FFFF. It is also an order of magnitude faster: Taking the square root of an integer takes between 600 and 700 cycles, whereas the EXEC's routine has a much wider range, taking nearly 10,000 cycles in some cases. In fact, the worst case performance for a square root is roughly a third faster than the worst case performance of FASTDIVU.

Where the code below refers to "Qpt" or "Q-point," this means the same as "fractional point" or "binary point."

# Source Code

;* ======================================================================== *; ;* These routines are placed into the public domain by their author. All *; ;* copyright rights are hereby relinquished on the routines and data in *; ;* this file. -- Joseph Zbiciak, 2008 *; ;* ======================================================================== *; ;; ======================================================================== ;; ;; NAME ;; ;; SQRT Calculate the square root of a fixed-point number ;; ;; SQRT.1 Calculate the square root of an integer ;; ;; SQRT.2 Calculate the square root of a fixed-point number ;; ;; ;; ;; AUTHOR ;; ;; Joseph Zbiciak <intvnut AT gmail.com> ;; ;; ;; ;; REVISION HISTORY ;; ;; 12-Sep-2001 Initial revision . . . . . . . . . . . J. Zbiciak ;; ;; 24-Nov-2003 Minor tweaks for speed . . . . . . . . J. Zbiciak ;; ;; ;; ;; INPUTS for SQRT ;; ;; R1 Unsigned 16-bit argument to SQRT() ;; ;; R5 Pointer to DECLE containing Qpt, followed by return addr. ;; ;; ;; ;; INPUTS for SQRT.1 ;; ;; R1 Unsigned 16-bit argument to SQRT() ;; ;; R5 Return address ;; ;; ;; ;; INPUTS for SQRT.2 ;; ;; R0 Qpt for fixed-point value ;; ;; R1 Unsigned 16-bit argument to SQRT() ;; ;; R5 Return address ;; ;; ;; ;; OUTPUTS ;; ;; R0 Zeroed ;; ;; R1 Unmodified ;; ;; R2 SQRT(R1) ;; ;; R3, R4 Unmodified ;; ;; R5 Trashed ;; ;; ;; ;; NOTES ;; ;; The way this code handles odd Q-points on fixed-point numbers is ;; ;; by right-shifting the incoming value 1 bit, thus making the ;; ;; Q-point even. This has the negative effect of losing precision ;; ;; on odd Q-point numbers. Rectifying this without losing any ;; ;; performance would require significantly larger codesize. ;; ;; ;; ;; CODESIZE ;; ;; 44 words ;; ;; ;; ;; CYCLES ;; ;; cycles = 139 + 71*(8 + Qpt/2) worst case for SQRT ;; ;; cycles = 121 + 61*(8 + Qpt/2) best case for SQRT ;; ;; ;; ;; Subtract 4 cycles if Qpt is even. ;; ;; Subtract 8 cycles if calling SQRT.1. ;; ;; Subtract 14 cycles if calling SQRT.2. ;; ;; ;; ;; SOURCE ;; ;; Loosely based on a C code example (mborg_isqrt2) by Paul Hseih and ;; ;; Mark Borgerding, found on the web here: ;; ;; ;; ;; http://www.azillionmonkeys.com/qed/sqroot.html ;; ;; ;; ;; Includes additional optimizations that eliminate some of the math. ;; ;; ======================================================================== ;; SQRT PROC MVI@ R5, R0 ; 8 Get Qpt from after CALL INCR PC ; 6 (skip CLRR R0) @@1: CLRR R0 ; 6 Set Qpt == 0 @@2: PSHR R5 ; 9 Alt entry point w/ all args in regs. PSHR R1 ; 9 save R1 PSHR R3 ; 9 save R3 ;---- ; 41 (worst case: SQRT) ; 33 (if SQRT.1) ; 27 (if SQRT.2) CLRR R2 ; 6 R2 == Result word MVII #$4000, R3 ; 8 R3 == 1/2 of square of test bit MOVR R3, R5 ; 6 R5 == bit * (2*guess + bit) INCR R0 ; 6 SARC R0, 1 ; 6 Check to see if Qpt is odd ; BC @@even_q ; 7/9 ADCR PC ; 7 SLR R1, 1 ; 6 Note: We lose LSB if odd Q @@even_q: ADDI #8, R0 ; 8 B @@first ; 9 ;---- ; 60 (worst case, q is ODD) ; 54 (q is EVEN) ;==== ; 83 (worst case: SQRT, q is ODD) @@loop: SLLC R1, 1 ; 6 Shift the value left by 1 BC @@b1 ; 7/9 MSB was 1, force guessed bit to 1. @@first: CMPR R5, R1 ; 6 Is (guess+bit)**2 <= val? BNC @@b0 ; 7/9 C==0 means the bit should be 0 @@b1: RLC R2, 1 ; 6 Yes: Set bit in result and SUBR R5, R1 ; 6 subtract guess from value ADDR R3, R5 ; 6 \ SLR R3, 1 ; 6 |-- Calculate next guess value ADDR R3, R5 ; 6 / DECR R0 ; 6 BNEQ @@loop ; 9/7 Guess next bit PULR R3 ; 11 Restore R3 PULR R1 ; 11 Restore R1 PULR PC ; 11 Return ;---- ; 71*k + 31 worst case @@b0: SLL R2, 1 ; 6 No: Clear bit in guessed result SLR R3, 1 ; 6 \__ Calculate next guess SUBR R3, R5 ; 6 / DECR R0 ; 6 BNEQ @@loop ; 9/7 Guess next bit ;---- ; 61*k - 2 best case @@done: PULR R3 ; 11 Restore R3 PULR R1 ; 11 Restore R1 PULR PC ; 11 Return ENDP ;; ======================================================================== ;; ;; End of file: sqrt.asm ;; ;; ======================================================================== ;;