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79 lines
2.8 KiB
79 lines
2.8 KiB
;
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;==================================================================================================
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; DECODE 32-BIT VALUES FROM A 5-BIT SHIFT-ENCODED VALUE
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;==================================================================================================
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;
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; Copyright (C) 2014 John R. Coffman. All rights reserved.
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; Provided for hobbyist use on the Z180 SBC Mark IV board.
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;
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; This program is free software: you can redistribute it and/or modify
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; it under the terms of the GNU General Public License as published by
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; the Free Software Foundation, either version 3 of the License, or
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; (at your option) any later version.
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;
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; This program is distributed in the hope that it will be useful,
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; but WITHOUT ANY WARRANTY; without even the implied warranty of
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; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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; GNU General Public License for more details.
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;
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; You should have received a copy of the GNU General Public License
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; along with this program. If not, see <http://www.gnu.org/licenses/>.
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;
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; THE FUNCTION(S) IN THIS FILE ARE BASED ON LIKE FUNCTIONS CREATED BY JOHN COFFMAN
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; IN HIS UNA BIOS PROJECT. THEY ARE INCLUDED HERE BASED ON GPLV3 PERMISSIBLE USE.
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;
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; An encoded value (V) is defined as V = C * 2^X * 3^Y
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; where C is a prearranged constant, X is 0 or 1 and Y is 0-15
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; The encoded value is stored as 5 bits: YXXXX
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; At present, C=75 for baud rate encoding and C=3 for CPU OSC encoding
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; DECODE
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;
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; Enter with:
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; HL = word to be decoded (5-bits) FXXXX
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; F=extra 3 factor, XXXX=shift factor, reg H must be zero
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; DE = encode divisor OSC_DIV = 3, or BAUD_DIV = 75
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;
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; Exit with:
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; DE:HL = decoded value
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; A = non-zero on error
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;
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decode:
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ld a,h ; set to test
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ld c,$ff ; presume error condition
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or a ; test for zero
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jr nz,decode9 ; not an encoded value
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ld a,l ; get low order 5 bits
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cp 32 ; test for error
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jr nc,decode9 ; error return if not below
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; argument hl is validated
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ld h,d
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ld l,e ; copy to hl
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cp 16
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jr c,decode2 ; if < 16, no 3 factor
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add hl,de ; introduce factor of 3
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add hl,de ; **
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decode2:
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ld de,0 ; zero the high order
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and 15 ; mask to 4 bits
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jr z,decode8 ; good exit
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ld c,b ; save b-reg
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ld b,a ;
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decode3:
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add hl,hl ; shift left by 1, set carry
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rl e
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rl d ; **
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djnz decode3
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ld b,c ; restore b-reg
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decode8:
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ld c,0 ; signal good return
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decode9:
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ld a,c ; error code test
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or a ; error code in reg-c and z-flag
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ret
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