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[1024MAK]

I don't mind that it has 0 as false and non-zero as true. But the unique and weird thing is the evaluation result of an AND and OR. Most languages and the QL will give a 1 for the result of and AND or OR but activate as true for any non-zero in an IF. For the ZX81 to return the first argument as true in AND and convert to a 1 for OR is inconsistent. At least if there was consistency I would expect 2 OR 17 to return 2 and 17 OR 2 to return 17, not that I like that. I'd prefer those results to all be 1. Whereas I'm fine with IF 17 THEN PRINT "HELLO" to work. This will need fixing on the ZXSimulator as a work-around :-/

Here is the relevant Z80 assembly for OR and AND...

Code: Select all

; -----------------
; THE 'OR' OPERATOR
; -----------------
; (offset: $07 'or' )
; The Boolean OR operator. e.g. X OR Y
; The result is zero if both values are zero else a non-zero value.
;
; e.g.    0 OR 0  returns 0.
;        -3 OR 0  returns -3.
;         0 OR -3 returns 1.
;        -3 OR 2  returns 1.
;
; A binary operation.
; On entry HL points to first operand (X) and DE to second operand (Y).

;; or
L351B:  EX      DE,HL           ; make HL point to second number
        CALL    L34E9           ; routine TEST-ZERO
        EX      DE,HL           ; restore pointers
        RET     C               ; return if result was zero - first operand, 
                                ; now the last value, is the result.

        SCF                     ; set carry flag
        JR      L350B           ; back to FP-0/1 to overwrite the first operand
                                ; with the value 1.


; ---------------------------------
; THE 'NUMBER AND NUMBER' OPERATION
; ---------------------------------
; (offset: $08 'no-&-no')
;   The Boolean AND operator.
;
;   e.g.    -3 AND 2  returns -3.
;           -3 AND 0  returns 0.
;            0 and -2 returns 0.
;            0 and 0  returns 0.
;
;   Compare with OR routine above.

;; no-&-no
L3524:  EX      DE,HL           ; make HL address second operand.

        CALL    L34E9           ; routine TEST-ZERO sets carry if zero.

        EX      DE,HL           ; restore pointers.
        RET     NC              ; return if second non-zero, first is result.

;

        AND     A               ; else clear carry.
        JR      L350B           ; back to FP-0/1 to overwrite first operand
                                ; with zero for return value.

Code copied from here.
See also The Complete Spectrum ROM Disassembly. You want page 208 (PDF page) / 203 (actual book page).

Mark