Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 39)

Microcomputer Architecture 25

memory. If an instruction causes a branch to a part of the program other

than the next sequential instruction, the PC will be updated automatically.

The final register in the user programmers model is the condition

code register, or CCR. This is an 8-bit register that contains individual

bits that are set or reset as the result of arithmetic instructions. The CCR

will be covered in detail in Chapter 5.

The supervisor programmer’s model is identical to the user mode

programmer’s model with two exceptions. First, in the supervisor mode

there is a different register A7, known as the supervisor stack pointer, or

SSP. This register is totally distinct from the USP. Second, the condition

code register is still present, but it is in a 16-bit form. Together with the

new high-order 8 bits, it is known as the status register, or SR. Figure 5

shows the programmer’s models for both the user and supervisor modes.

n p r Program

J Counter

" 1 | Condition Code

___________

lCCR Register

User Programmer's Model

____________________

1615

_____________________

; IA7’ Supervisor Stack

----------------------------------------1 J(SSP) Pointer

_________

8 7

__________

j CCR |sR Status Register

Supervisor Programmer’s Model Supplement

Figure 5 Programmer's models. (Courtesy of Motorola, Inc.)

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 255 256

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Motorola M68000 Betriebsanweisung Seite 39