1. xerox 560 computer system - The UK Mirror Service

eference address field of the instruction is used toobtain a word, and the 17 or 20 low-order bits of theword thus obtained effectively replace the initial referenceaddress field; then indexing is carried out accordingto the operation code of the instruction. SeeFigures 7 and 9, later in this chapter.6. Index Reference Address. If indexing is called for inthe instruction (a value other than zero in bits 12-14of the instruction), the direct or indirect reference addressis modified by addition of the displacement valuein the general register (index) called for by the instruction(after scaling the displacement according to theinstruction type). This final reference address value(after indirect addressing, indexing, or both) is definedas the effective virtual address of the instruction. Indexingafter indirect addressing is ,ca lied postindexing.See also Figures 7 and 9, later in this chapter.7. Displacements. Displacements are the 16- to 22-bitvalues used in index registers and by byte-string instructionsto generate effective addresses of the appropriatesize (byte, halfword, word, or doubleword).8. Register Address. If any instruction provides a virtualaddress that is a memory reference (i .e., a direct,indirect, or indexed reference address) in the range 0through 15, the basic processor does not attempt to readfrom or write into main memory locations 0 through 15.Instead, the four low-order bits of the reference addressare used as a general register address and the generaI reg ister correspond i ng to th is address is used as theoperand location or result destination. Thus, the instructioncan use any of the first 16 registers in the currenTregisTer biocK as Tne source or an operand, thelocation of a direct address, or the destination of a result.Such usage is called a "register-to-register"operation.9. Actual Address. This is the address value actually usedby the basic processor to access main memory via thememory address register (see Figure 5). If the effectivevirtual address is in the range 0 through 15 (X10 throughX'F '), one of the first 16 general registers in the currentregister block is being addressed. If the basic processoris operating in the virtual addressing mode, alladdresses grea ter than 15 (X 1 F I) are transformed (usua IIyinto addresses in a different memory page) by the memorymap into actual addresses. Contrarily, if the basicprocessor is operating in either real or real extendedmode, no transformation via the memory map takes place.10. Effective Address. The effective address is defined asthe final virtual address computed for an instruction.Note, however, that some instructions do not use theeffective address as a location reference; instead, theeffective address is used to control the operation ofthe instruction (as in a shift instruction), to designatethe address of an input/output device (as in an input/output instruction), or to designate a specific elementof the system (as in a READ DIRECT or WRITE DIRECTinstruction) •11. Effective Location. An effective location is definedas the actual location (in main memory or in the currentregister block) that is to receive the result of a memoryreferencinginstruction, and is referenced by means ofan effective address. Because an effective addressmay be either an actual address or a virtual address,when applicable, this definition of an effective locationassumes the transformation of a virtual address intoan actual address.12. Effective Operand. An effective operand is definedas the contents of an actual location (in main memoryor in the current register block) that is to be used asan operand by a memory-referencing instruction, andis referred to by means of an effective address. Thisalso presupposes the transformation of a virtual addressinto an actual address.TYPES OF ADDRESSINGExcept for the special type of addressing performed by someinterrupt and trap instructi ons, all addressing within thecomputer system is real, real extended, or virtual.REAL ADDRESSINGIn real addressing, a one-to-one relationship prevails betweenthe effective virtual address of each instructionand the actual address used to access main memory. Realaddressing has these characteristics:1. Each reference address is a 17-bit word address.2. The reference address may be direct or indirect, withor without postindexing.3. Displacements associated with indexing are automaticallyaligned, as required, using the full 32-bit contentsof the index register. The final result is truncated tothe left of the high-order bit of the original 17-bit referenceaddress, and the effective real address is a16-bit doubleword address, 17-bit word address, 18-bithalfword address, or a 19-bit byte address.4. If indirect addressing is invoked, the 17-bit referenceaddress in the instruction word is used to access the indirectaddress word in memory. The low-order 17 bitsof this word then replace the reference address of theinstruction word in the calculations described in (3),above.5. Memory mapping and memory access protection arenever invoked.6. Memory write protection is automatically invoked.7. Leading zeros are automatically appended to the effectiveaddress to generate an actual word address as requiredby the main memory.Main Memory 19