Zilog Z16F2810 Uživatelský manuál stáhnout pdf (Strana 47)

UM018809-0611 Operand Addressing

ZNEO

CPU Core

User Manual

Resizing Data

When an 8-bit or 16-bit memory location is written, the value from the source register is

truncated, so only the least-significant 8 or 16 bits of the register value are written, respec-

tively. The source register itself is not changed. When an 8-bit or 16-bit memory location

is read, the value from memory must be extended to a full 32 bits before it is used or stored

in a register.

One of the following two kinds of data extension must be used:

Unsigned (Zero) Extension. The upper bits of the new 32-bit value are filled with zeroes.

Unsigned extension is invoked by including a ‘U’ in the mnemonic suffix. For example,

the following instruction loads the byte at

FFFF_7002H into R10[7:0] and fills R10[31:8]

with zeroes:

LD.UB R10,7002H

Signed Extension.

The upper bits of the new 32-bit value are filled with ones or zeroes,

depending on the source value’s most-significant (sign) bit, to preserve the sign of the

loaded value. Signed extension is invoked by including an ‘S’ in the mnemonic suffix.

For example, the following instruction loads the byte at address

FFFF_7002H into register

bits R10[7:0] and copies bit 7 of that byte into each bit of R10[31:8].

Figure 7. Mapping of Register to Memory Bytes

ALU Registers Memory Space

7:015:823:1631:24Bits:

FFFF_7005H

(Effective Address)

FFFF_7006H

FFFF_7007H

FFFF_7004H

7:015:823:1631:24Bits:

FFFF_7009H

(Effective Address)FFFF_7008H

7:015:823:1631:24Bits:

FFFF_700AH (Effective Address)

8-Bit

16-Bit

32-Bit

1 2 ... 42 43 44 45 46 47 48 49 50 51 52 ... 215 216

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Zilog Z16F2810 Uživatelský manuál Strana 47