TABLE 87.2 Addressing Modes 1. Explicit addresses Example 1. 1. Absolute addressing 765 The actual address written into the instruction 1. 2. Register indirect 3) Meaning"the address is in register 3. 1.3. Base-displacement -12(r3) Meaning"12 bytes before the address in register 3. 14. Baseindex (r3, r4) Meaning make an address by adding the contents of r3 and r4. This mode has many riations which are discussed below 1.5. Double indirect @5(r4) Very uncommon! Means calculate an address as in 1.3, then fetch the longword there, and then use it as the address of what you really want. 2. Direct data specification 2.1. Immediate/literal 6 or 6 Meaning"use 6 as the datum. "In machines which use #6, 6 without# means address 6. This is called"absolute addressing. 3. Program-relative 3.1. Labels loop: The label (typically an alphanumeric ending in a colon)is a marker in the program which the assembler and linker keep track of. The common uses are to jump to a labeled spot or to load labeled constants stored with the program. 4. Address-modifying form 4. 1. Postincrement (sp) Same as 1. 2 except that, after the address is used, it is incremented by the size of the datum in bytes and returned to the register from which it came. 4.2. Predecrement -(sp) The value in SP is decremented by the size of the datum in bytes, used as the address and returned to the register from which it came CVTWL (r4)[r5], (r6)r5] i convert the words starting at(r4)to longwords starting at(r6) Note that the same index, [r5], is used for both arrays. On the left, the contents of r5 are multiplied by 2 and added to r4 to get the address; on the right, the address is r5*4+r6. You would be saying: Convert the 4th word to the 4th longword. This is undoubtedly compact and sometimes convenient. It is also unique to the VAX. For the 68000, the designers folded both base-displacement and base- index into one mode and made room for word or longword indices. It looks like dress =(A3+64)+sign-extended(D2) The 68000 limits the displacement to a signed byte, but other than that, it is indeed a rather general indexing format. If you do not want the displacement, set it to O For the powerful but simple SPARC, the simple base-index form shown in 1. 4 is all that you have (or need The double- indirect format, 1.5, is so rarely used that it has been left out of almost all designs but the VAX. What makes it occasionally useful is that subroutines get pointers to"pass by pointer"variables. Thus, if you want to get the variable, first you must load the address and then the variable. The vaX allows you to do this one instruction. While that sounds compact, it is expensive in memory cycles. If you want to use that pointer again, it pays to have it in register. The two items under heading 4 are strange at first. Their principal function is adding items to and removing them from a dynamic stack, or for C, to execute the operation *X++ or*(X). The action may be viewed with the code below and the illustration of memory in Fig. 87.2 movl r4,-(sp) make room on the stack(subtract 4 from SP)and put movl(sp)+, r4 stake a longword off the stack, shorten the stack by 4 bytes, and put the longword in r4 RISCs abhor instructions which do two unrelated things. Instead of using a dynamic stack, they use a quasi static stack. If a subroutine needs 12 bytes of stack space, it explicitly subtracts 12 from SP. Then it works from there with the base-displacement format(1.3)to reference any place in the block of bytes just defined. If you want to use a pointer and then increment the pointer, RISCs will do that as two independent instructions. Let us consider one short section of MC68000 code from our sample program in C to see how these modes work and to sample some of the flavor of the language e 2000 by CRC Press LLC© 2000 by CRC Press LLC CVTWL (r4)[r5],(r6)[r5] ;convert the words starting at (r4) to longwords starting at (r6) Note that the same index, [r5], is used for both arrays. On the left, the contents of r5 are multiplied by 2 and added to r4 to get the address; on the right, the address is r5*4+r6. You would be saying: “Convert the 4th word to the 4th longword.” This is undoubtedly compact and sometimes convenient. It is also unique to the VAX. For the 68000, the designers folded both base-displacement and base-index into one mode and made room for word or longword indices. It looks like: add.1 64(A3,D2.w),D3 ;address = (A3+64) +sign-extended(D2) The 68000 limits the displacement to a signed byte, but other than that, it is indeed a rather general indexing format. If you do not want the displacement, set it to 0. For the powerful but simple SPARC, the simple base-index form shown in 1.4 is all that you have (or need). The double-indirect format, 1.5, is so rarely used that it has been left out of almost all designs but the VAX. What makes it occasionally useful is that subroutines get pointers to “pass by pointer” variables. Thus, if you want to get the variable, first you must load the address and then the variable. The VAX allows you to do this in one instruction. While that sounds compact, it is expensive in memory cycles. If you want to use that pointer again, it pays to have it in register. The two items under heading 4 are strange at first. Their principal function is adding items to and removing them from a dynamic stack, or for C, to execute the operation *X++ or *(– –X). The action may be viewed with the code below and the illustration of memory in Fig. 87.2: movl r4, –(sp) ;make room on the stack (subtract 4 from SP) and put the contents of r4 in that spot movl (sp)+, r4 ;take a longword off the stack, shorten the stack by 4 bytes, and put the longword in r4 RISCs abhor instructions which do two unrelated things. Instead of using a dynamic stack, they use a quasi￾static stack. If a subroutine needs 12 bytes of stack space, it explicitly subtracts 12 from SP. Then it works from there with the base-displacement format (1.3) to reference any place in the block of bytes just defined. If you want to use a pointer and then increment the pointer, RISCs will do that as two independent instructions. Let us consider one short section of MC68000 code from our sample program in C to see how these modes work and to sample some of the flavor of the language: TABLE 87.2 Addressing Modes 1. Explicit addresses Example 1.1. Absolute addressing 765 The actual address written into the instruction. 1.2. Register indirect (r3) Meaning “the address is in register 3.” 1.3. Base-displacement –12(r3) Meaning “12 bytes before the address in register 3.” 1.4. Base-index (r3,r4) Meaning make an address by adding the contents of r3 and r4. This mode has many variations which are discussed below. 1.5. Double indirect @5(r4) Very uncommon! Means calculate an address as in 1.3, then fetch the longword there, and then use it as the address of what you really want. 2. Direct data specification 2.1. Immediate/literal #6 or 6 Meaning “use 6 as the datum.” In machines which use #6, 6 without # means address 6. This is called “absolute addressing.” 3. Program-relative 3.1. Labels loop: The label (typically an alphanumeric ending in a colon) is a marker in the program which the assembler and linker keep track of. The common uses are to jump to a labeled spot or to load labeled constants stored with the program. 4. Address-modifying forms (CISC only) 4.1. Postincrement (sp)+ Same as 1.2 except that, after the address is used, it is incremented by the size of the datum in bytes and returned to the register from which it came. 4.2. Predecrement –(sp) The value in SP is decremented by the size of the datum in bytes, used as the address and returned to the register from which it came