A formal description of SYSTEM/360

in the case of ZAP (zero and add) and CVR (convert to binary), convertby using the single argunmlt k, directly. CVD also involves a single (lines j0-55argument only (j 1).The argument ki is determined (j19) as the signed base-10value of the vector al/u of decimal digits, u being assembled (j 15)by catenating the hasc-2 values of half-bytes from field i. Thelast half-byte determines the sign (jig), a negative sign beingrepresented by 13 (that is, 1101) in the extended RCD code(& = 0), and by 11 in the Anlerican Standard code. For example,if p12 = 0, I, = 1 and the bytes at Ma' and Ma1" are 0101 0000and 1001 1101, respectively, thcn u = 5,0, 9, 13, and k, = -509.The result X: is converted to thc decimal representation andstored (j34-42) and, except for DIJ (divide decimal), the operationsend on line j43. Because of the setting of j on line j33, thequotient k is stored only in the first (I, + 1) - (1, + 1) bytes ofthe first field, leaving space of length (I, + 1) for the remainder,which is computed on line j47 and converted and stored by repeatingfrom line j3.5.CVD produces an %byte result (j2), and CVD and CVB producespccification exceptions (j39, 3) if the argument address is notat an %byte boundary. E'or MP and DP, a specification exceptionis occasioned (j9) if the length of the second field exceeds either8 bytes or the length of the first (i.c., the result) field. All specificationexceptions suppress the instruction; except for CVD, inwhich the result field remains unchanged, all other errors makethe relevant result fields unreliable (348-55). A data exception f7occurs in ZAP (j8) if the fields overlap such that the right-handend of the second field is to the right of the right-hand end of thefirst. Data exceptions caused by improper overlapping in theother instructions are dctccted and entered on line j18.Two floating-point representations are used; the short (one- floating-pointword) and long (two-word) representations utilize logical vectors arithmeticu of dimensions 32 and 64, respectively. A number n, represented (lines k0-73)by u, is evaluated as follows:uo is the sign (0 for +, and 1 for -);I d/a"/u is the characteristic c;2"""* x 1 (Ys/u is the fraction f;c - 64 is the exponent e; and1 7 2 is equal to f X 16".The program comprises three ~najor segments, the fetchingof operands u and v (lines kO-l6), the conlputation of the results(k17-65), and the storing of results and the setting of the conditioncode and of (lost) significance ( ilr) and exponent underflow( fI3) exceptions (1~66-73).Floating point register i is selected by the address 2xi,and the relevant addresses (both u1 and u2 in the RR format)are therefore subject to a specification exception check (kl)which suppresses the instruction. Specification exceptions occasionedby MAC also suppress the instruction (1~14).IFORMAL DESCRIPTION OF SYSTEM/36O 217