Maxim Integrated MAX31782 - Manual (Page 182)

MaximIntegrated 19-17
MAX31782 User’s Guide
Revision 0; 8/11
Either of the data pointers may be post-incremented or post-decremented following any read or may be pre-increment-
ed or predecremented before any write access by using the following syntax .
move A[0], @DP[0]++ ; increment DP[0] after read
move @++DP[0], A[1] ; increment DP[0] before write
move A[5], @DP[1]-- ; decrement DP[1] after read
move @--DP[1], #00h ; decrement DP[1] before write
The Frame Pointer (BP[OFFS]) is actually composed of a base pointer (BP) and an offset from the base pointer (OFFS) .
For the frame pointer, the offset register (OFFS) is the target of any increment or decrement operation . The base pointer
(BP) is unaffected by increment and decrement operations on the Frame Pointer . Similar to DP[n], the OFFS register
may be preincremented/decremented when writing to data memory and may be post-incremented/decremented when
reading from data memory .
move A[0], @BP[OFFS--] ; decrement OFFS after read
move @BP[++OFFS], A[1] ; increment OFFS before write
All three data pointers support both byte and word access to data memory . Each data pointer has its own word/byte
select (WBSn) special-function register bit to control the access mode associated with the data pointer . These three
register bits (WBS2, which controls BP[OFFS] access; WBS1, which controls DP[1] access; and WBS0, which controls
DP[0] access) reside in the Data Pointer Control (DPC) register . When a given WBSn control bit is configured to 1, the
associated pointer is operated in the word access mode . When the WBSn bit is configured to 0, the pointer is operated
in the byte access mode . Word access mode allows addressing of 64kWords of memory while byte access mode allows
addressing of 64kBytes of memory .
Each data pointer (DP[n]) and Frame Pointer base (BP) register is actually implemented internally as a 17-bit register
(e .g ., 16:0) . The Frame Pointer offset register (OFFS) is implemented internally as a 9-bit register (e .g ., 8:0) . The WBSn
bit for the respective pointer controls whether the highest 16 bits (16:1) of the pointer are in use, as is the case for word
mode (WBSn = 1) or whether the lowest 16 bits (15:0) are in use, as will be the case for byte mode (WBSn = 0) . The
WBS2 bit also controls whether the high 8 bits (8:1) of the offset register are in use (WBS2 = 1) or the low 8 bits (7:0)
are used (WBS2 = 0) . All data pointer register reads, writes, auto-increment/decrement operations occur with respect
to the current WBSn selection . Data pointer increment and decrement operations only affect those bits specific to the
current word or byte addressing mode (e .g ., incrementing a byte mode data pointer from FFFFh does not carry into
the internal high order bit that is utilized only for word mode data pointer access) . Switching from byte to word access
mode or vice versa does not alter the data pointer contents . Therefore, it is important to maintain the consistency of
data pointer address value within the given access mode .
move DPC, #0 ; DP[0] in byte mode
move DP[0], #2345h ; DP[0]=2345h (byte mode)
; internal bits 15:0 loaded
move DPC, #4 ; DP[0] in word mode
move DP[0], #2345h ; DP[0]=2345h (word mode)
; internal bits 16:1 loaded
move DPC, #0 ; DP[0] in byte mode
move GR, DP[0] ; GR = 468Bh (looking at bits 15:0)
The three pointers share a single read/write port on the data memory and thus, the user must knowingly activate a
desired pointer before using it for data memory read operations . This can be done explicitly using the data pointer
select bits (SDPS1:0; DPC .1:0), or implicitly by writing to the DP[n], BP, or OFFS registers as shown below . Any indirect
memory write operation using a data pointer will set the SDPS bits, thus activating the write pointer as the active source
pointer .
move DPC, #2 ; (explicit) selection of FP as the pointer
move DP[1], DP[1] ; (implicit) selection of DP[1]; set SDPS1:0=01b
move OFFS, src ; (implicit) selection of FP; set SDPS1=1
move @DP[0], src ; (implicit) selection of DP[0]; set SDPS1:0=00b