UPD78C18 datasheet(16/110 Pages) NEC

16 / 110 page

µPD78C17,78C18

INTERNAL BLOCK FUNCTIONS

2.1

REGISTERS

The central registers are the sixteen 8-bit registers and four 16-bit registers shown in Fig. 2-1.

Fig. 2-1 Register Configuration

(a)

General registers (B, C, D, E, H, L)

There are two sets of general registers (MAIN: B, C, D, E, H, L; ALT: B’, C’, D’, E’, H’, L’). They function

as auxiliary registers for the accumulator, and have a data pointer function as register pairs (BC, DE, HL;

B’C’, D’E’, H’L’). In particular, four register pairs DE, D’E’, HL, and H’L’, have a base register function.

When the two sets are used, if an interrupt occurs in one set, the register contents are saved into the

other register set without saving them into the memory so that interrupt servicing can be carried out. The

other set of registers can also be used as data pointer expansion registers. Two addressing modes, single-

step automatic increment/decrement modes and a two-step automatic increment mode, are available for

the register pairs, DE, HL, D’E’, and H’L’, so that the processing time can be reduced. BC, DE, and HL can

be simultaneously replaced with the ALT register by means of the EXX instruction. The HL register can be

independently replaced with the ALT register by means of the EXH instruction.

(b) Working register vector register (V)

When a working area is set in the memory space, the high-order 8 bits of the memory address are

selected using the V register and the low-order 8 bits are addressed by the immediate data in the instruc-

tion. Thus, the memory area specified with the V register can be used as working registers with a 256 x 8-

bit configuration.

Because a working register can be specified with a 1-byte address field, program reduction is possible

by using the working area for software flags, parameters, and counters. The V register can be replaced

with the ALT register paired with an accumulator by means of the EXA instruction.

EA'

MAIN

ALT

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