Texas Instruments MSP50C614 manual Rflag

Bit, Byte, Word and String Addressing

Example 4.5.7	MOV STR, 4±2
	MOV	AP0, 2
	MOV	R0, 0x0001 * 2
	MOVBS A0, *R0++

Refer to Figure 4±4 for this example. The word-string length is 4. AP0 points to AC2 accumulator. R0 is loaded with 0x0002. The fourth instruction loads the value of the word-string at the RAM address in R0, 0x0002. R0 autoincrements by 2 after each fetch and stores them into four consecutive accumulators starting from AC2. The result is, AC2 = 0x5678, AC3 = 0x9ABC, AC4 = 0xDEF0, AC5 = 0x1122. There were 4 word fetches and the new value of R0 = 0x000A.

Example 4.5.8	SFLAG		*0x0003
	MOV	A0,		*0x0003	*	2
	RFLAG		*0x0003
	MOV	A0,		*0x0003	*	2

Refer to Figure 4±4 for this example. This example illustrates the use of the TAG and flag bits. Notice that SFLAG uses a word address, 0x0003, while the MOV instruction uses a byte address 0x0003 * 2. The first instruction sets the flag/tag bit at flag address 0x0003. Flag address 0x0003 represents the 17th bit of the 3rd word (or 6th byte) of RAM. In the second instruction, this flag bit is placed in the TAG status bit of the STAT and the value in RAM location 0x0003 * 2 is placed in A0. The third instruction resets the flag/tag to 0 at the same flag address. The fourth instruction reads the same word memory loca- tion and writes the TAG bit of STAT, which is now 0. Note: SFLAG *0x0003 could have been replaced by STAG *0x0003 * 2 and RFLAG *0x0003 could have been replaced by RTAG *0x0003 * 2.

Example 4.5.9	SFLAG *0x0005
	MOVB A0, *0x000b
	RFLAG *0x0005

MOVB A0, *0x000b

Refer to Figure 4±4 for this example. The SFLAG instruction sets the 17th bit (tag/flag) of the 5th word of RAM. The MOVB instruction gets the lower byte of the 5th word of RAM and puts it in A0. In addition, the TAG bit of the STAT register is set. If the MOVB instruction addressed *0x000A instead of *0x000B, the STAT register would still be updated with the same tag/flag bit (the 17th bit of the 5th word of RAM). This means that odd byte locations in RAM, RAModd, have the same tag/flag as the preceding byte location RAModd ±1. For exam- ple, the 7th word of RAM is made up of two bytes: 0x000E, and 0x000F. These two byte locations share the same tag/flag bit.

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