IBM System/7 Functional Characteristics - All about the IBM 1130 ...

PARITYA System/7 word, whether used to hold data or an instruction, consists of 16 informationbits as shown in Figure 2-3. Every 16-bit word (bits 0 to 15) contains two 8-bit bytes(bits 0 to 7 and 8 to 15). Each byte may have an odd number of bits set on (that is, havinga value of 1), called odd parity. Obviously, this does not occur at all times becausedata or instructions can be represented with an even number of bits set on. For example,00000000 00001001 has no bits set on in the first byte and two bits set on in thesecond byte. (This is an example of even parity.)0 7 8 1511 1 1 1 I I I I 1 1 1 I IBit 0 Bit 15Byte 1 Byte 2Figure 2-3. System/7 storage wordTo ensure internal accuracy in data transmission and manipulation, the ability tocheck for odd parity is designed into the System/7 by adding two parity bits (P0 andP1) to each word. (So, in reality, each word in processor storage contains 18 bits.)Each parity bit is associated only with its corresponding byte as shown by the followingbit organization of a System/7 word:Po 0 1 2 3 4 5 6 7 P1 8 9 10 11 12 13 14 15Each parity bit is used to maintain odd parity with its associated byte by thefollowing method. If the 8-bit byte already has odd parity, its parity bit is set to a0 value. If the 8-bit byte has even parity, its parity bit is set to a 1 value to givethe entity (byte plus parity bit) odd parity. The previous example of even parity byteswould appear in main storage as:Po P11 00000000 1 00001001Any time that the value (contents) of a word is intentionally changed during programexecution, the machine automatically modifies the parity bits to maintain odd parity.Every time a word is used in the system, each byte plus its parity bit is checked toensure that an odd number of bits is set on. If an even number of bits is detected in theprocessor (a parity error), a machine check error occurs. A parity error detected withinthe direct control channel or I/O modules is recorded in a status indicator word (forexample, device status word or direct control channel status word).Words entering the System/7 from a host processor are analyzed to determine whethereach byte has odd or even parity. Parity bits are then added to the word to maintain oddparity.2-6 GA34-0003