Maxim Integrated MAXQ Family - Manual (Page 48)

4-2
MAXQ Family User’s Guide
SECTION 4: SYSTEM REGISTER DESCRIPTIONS
Those registers currently defined in the MAXQ System Register map are described in the following pages. The addresses for each reg-
ister are given in the format module[index], where module is the module specifier from 8h to Fh and index is the register sub-index
from 0h to Fh.
4.1 Accumulator Pointer Register (AP, 8h[0h])
The bit definitions are for 16 accumulators.
Initialization: This register is cleared to 00h on all forms of reset.
Access: Unrestricted direct read/write access.
4.2 Accumulator Pointer Control Register (APC, 8h[1h])
The bit definitions are for 16 accumulators.
Initialization: This register is cleared to 00h on all forms of reset.
Access: Unrestricted direct read/write access.
BIT FUNCTION
AP.3 to AP.0
Active Accumulator Select. These bits select which of the 16 accumulator registers are used for arithmetic and logical operations.
If the APC register has been set to perform automatic increment/decrement of the active accumulator, this setting will be
automatically changed after each arithmetic or logical operation. If a ‘MOVE AP, Acc’ instruction is executed, any enabled AP
inc/dec/modulo control will take precedence over the transfer of Acc data into AP.
AP.7 to AP.4 Reserved. All reads return 0.
BIT FUNCTION
Accumulator Pointer Auto Increment/Decrement Modulus. If these bits are set to a nonzero value, the accumulator pointer (AP[3:0])
will be automatically incremented or decremented following each arithmetic or logical operation. The mode for the auto-increment/
decrement is determined as follows:
MOD[2:0] AUTO INCREMENT/DECREMENT MODE
000 No auto-increment/decrement (default)
001 Increment/decrement AP[0] modulo 2
010 Increment/decrement AP[1:0] modulo 4
011 Increment/decrement AP[2:0] modulo 8
100 Increment/decrement AP modulo 16
APC.2 to APC.0
(MOD2 to
MOD0)
101 to 111 Reserved (modulo 16 when set)
APC.5 to APC.3 Reserved. All reads return 0.
APC.6 (IDS)
Increment/Decrement Select. If this bit is set to 0, the accumulator pointer AP is incremented following each arithmetic or logical
operation according to MOD[2:0]. If this bit is set to 1, the accumulator pointer AP is decremented following each arithmetic or
logical operation according to MOD[2:0]. If MOD[2:0] is set to 000, the setting of this bit is ignored.
APC.7 (CLR)
AP Clear. Writing this bit to 1 clears the accumulator pointer AP to 0. Once set, this bit will automatically be reset to 0 by hardware.
If a ‘MOVE APC, Acc’ instruction is executed requesting that AP be set to 0 (i.e., CLR = 1), the AP clear function overrides any
enabled inc/dec/modulo control. All reads from this bit return 0.
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