T 18 Little Endian (SPARC, MC68000) FIGURE 87.3 Byte numbering and number placement for big- and little-endian systems. Hexadecimal numbers are used memory addresses 0 01020304 18 1B Big Endian Little Endian (SPARC, MC68000) (VAX) FIGURE 87.4 The same items as in Fig. 87.3, but with justification of the long integers to begin on a longword bound it is all just a list of bytes. You may access them two at a time, four at a time, or one at a time. Any interpretation of those bytes is entirely up to the program. Unlike the HLL which requires that you tell it what each named variable is, assembly language knows only bytes and groups of bytes. In assembly language, the"T"can be thought of as a letter or the number 54(HEX). Your choice. Or, more importantly, your programs choice Addressing Now that we have both memory and addresses, we should next consider how these processors require that programmers specify the data that is to be acted upon by the instructions All of these machines have multiple modes of address. The VAX has the biggest vocabulary; the SPARC the smallest. Yet all can accomplish the same tasks. Four general types of address specification are quite common mong assembly languages. These are shown in Table 87. 2. They are spelled out in words in the table, but their usage is really developed in the examples which follow in this and the succeeding sections. In Table 87. 2, formats 1.4 and 1.5 and the entries in 4 require some expansion. The others will be clear in ne examples we will present. Base-index addressing is the mechanism for dealing with subscripts. The base ints at the starting point of a data structure, such as a string or a vector; the index measures the offset from the start of the structure to the element in question. For most machines, the index is simply a separate regist which counts the bytes from the base to the item in question. If the items in the list are 4 bytes long, then to increment the index, you add 4. While that is not hard to remember, the vax does its multiplication by the item length for you. Furthermore, it allows you to index any form of address that you can write. To show you what that means, consider expanding numbers stored in words into numbers stored in longwords. The extension is to preserve sign. The VAX provides specific instructions for conversions. If we were moving these words in one array to longwords in another array, we would write: e 2000 by CRC Press LLC© 2000 by CRC Press LLC it is all just a list of bytes. You may access them two at a time, four at a time, or one at a time. Any interpretation of those bytes is entirely up to the program. Unlike the HLL which requires that you tell it what each named variable is, assembly language knows only bytes and groups of bytes. In assembly language, the “T” can be thought of as a letter or the number 54 (HEX). Your choice. Or, more importantly, your program’s choice. Addressing Now that we have both memory and addresses, we should next consider how these processors require that programmers specify the data that is to be acted upon by the instructions. All of these machines have multiple modes of address. The VAX has the biggest vocabulary; the SPARC the smallest. Yet all can accomplish the same tasks. Four general types of address specification are quite common among assembly languages. These are shown in Table 87.2. They are spelled out in words in the table, but their usage is really developed in the examples which follow in this and the succeeding sections. In Table 87.2, formats 1.4 and 1.5 and the entries in 4 require some expansion. The others will be clear in the examples we will present. Base-index addressing is the mechanism for dealing with subscripts. The base points at the starting point of a data structure, such as a string or a vector; the index measures the offset from the start of the structure to the element in question. For most machines, the index is simply a separate register which counts the bytes from the base to the item in question. If the items in the list are 4 bytes long, then to increment the index, you add 4. While that is not hard to remember, the VAX does its multiplication by the item length for you. Furthermore, it allows you to index any form of address that you can write. To show you what that means, consider expanding numbers stored in words into numbers stored in longwords. The extension is to preserve sign. The VAX provides specific instructions for conversions. If we were moving these words in one array to longwords in another array, we would write: FIGURE 87.3 Byte numbering and number placement for big- and little-endian systems. Hexadecimal numbers are used for the memory addresses. FIGURE 87.4 The same items as in Fig. 87.3, but with justification of the long integers to begin on a longword boundary