Motorola M68000 User's Guide download pdf (Page 192)

178 Assembly Language Programming for the 68000 Family

instructions to be used in conjunction with the function class output lines.

A source function code register, SFC, and a destination function code

bits each, each bit corresponding to a bit of the function class lines. A new

privileged instruction, the move to/from control register, is provided on

the 68010. Its general form is:

MOVEC Rc,Rn

MOVEC Rn,Rc

Rc = SFC, DFC, VBR, USP

Rn = D0-D7, A0-A7

It is always a 32-bit transfer, with unused bits read as zero. This instruction

can also be used to access the user stack pointer, USP, or the vector base

relevant value.

A second privileged instruction, move to/from address space, can now

be used to access the memory location in the address space specified

by the source or destination function registers. The general form of this

instruction is:

MOVESI.<size>] Rn,<ea>

MOVESI.<size>] <ea>,Rn

<size> = B, W, L

Rn = D0-D7, A0-A7

The SFC or DFC is used, as appropriate, depending on whether <ea>

is the source or destination of the instruction.

The Vector Base Register

You will recall from Chapter 12 that for the 68000 the exception

vectors start at memory location 016 and continue through 3FF10. This

is normally the case for the 68010 as well. However, the 68010 provides a

method of relocating the exception vectors to any place desired. A special

32-bit register is initially set to zero upon a system reset. A program

running in the supervisor mode can change the contents of this register

by use of the MOVEC instruction described above. The contents of the

VBR is always added to the address that would normally be used to

process the exception. You may recall that this address is four times the

exception number. For example, the TRAP #0 instruction will generate

exception number 3210. The actual vector location will be at 12810 or 8016.

If the VBR contains 400016, the TRAP #0 exception vector is located at

408016. The following instructions can be used to set the VBR to 400016:

1 2 ... 187 188 189 190 191 192 193 194 195 196 197 ... 255 256

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Motorola M68000 User's Guide Page 192