Motorola M68000 User's Guide download pdf (Page 207)

The 68020 193

format that must be used. For example, to sign-extend the byte in DO,

you would issue an EXTB.L DO instruction.

The LINK instruction was formally restricted to use a sign-extended

16-bit displacement to offset the stack pointer. The 68020 allows a full

32-bit offset to be added to the stack pointer.

Hew Instructions

The 68020 has a relatively small number of totally new instructions.

Since these instructions are fairly well spread out over the overall func

tionality of the 68020,1 will cover them in alphabetical order.

Bit Field Instructions BFxxx The first new instruction is actually a group

of instructions. They each manipulate a bit field. A bit field is a group

of 1 to 32 contiguous bits in either a register or memory. The bit field

is specified by a field width and offset. The assembler language format

recommended by Motorola is {offset:width}. Offset can be an immediate

value of 0 to 31 or a value in a data register. If a data register is used,

it has a range of —231 to 231—1. The offset is the bit offset from the

high-order bit. In other words, an offset of 0 specifies a bit field starting

at bit number 31. Width can be an immediate value of 0 to 31 or a value

in a data register. If a data register is used, the value is taken modulo 32.

In either case, a value of 0 represents a field width of 32. The specific

assembler syntax can vary, so be sure to check your assembler manual.

This is especially important concerning whether an immediate field width

of 32 is allowed and automatically converted to 0.

Here is a list of the bit field instructions and their functions:

BFCBG <ea>(offset:width)

BFCLR <ea>(offset:width)

BFEXTS <ea>(offset:width),Dn

BFEXTU <ea>(offset:width),Dn

BFFFO <ea>(offset:width),Dn

BFINS Dn,<ea>(offset:width)

BFSET <ea>(offset:width)

BFTST <ea>(offset:width)

One's complement the bit field

Clear the bit field

Bit field->Dn, sign extended

Bit field->Dn, zero extended

Search for first bit set, offset->Dn

Dn->bit field

Set all bits to one

Set N and Z conditions, treat the

bit field as a signed number

Breakpoint Instruction BKPT The BKPT (breakpoint) instruction is

provided for use by debuggers and hardware emulators. In order for

this instruction to be used, external hardware is required. When the

BKPT instruction is executed, a special bus cycle is executed. The BKPT

instruction has a single immediate mode operand. This operand is placed

on address line A2-A4. The immediate value must be in the range 0-7.

If external hardware is present, it can respond with an instruction word

1 2 ... 202 203 204 205 206 207 208 209 210 211 212 ... 255 256

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Motorola M68000 User's Guide Page 207

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