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ST9 USER GUIDE

3.1.7.1 Program Segment

We can consider this memory as linear since we can jump anywhere in memory space using

the special JPS, CALLS and RETS instructions.

The 6-bit CSR register is used to extend the 16-bit address to a 22-bit address by concatena-

tion (see Figure 10). To make a fast branch in the same segment, use the common JP, CALL

and RET instructions.

To branch to another segment using a far call, the use of CALLS saves the current PC value

and the CSR value (Code Segment Register) in the stack, before loading the PC and the CSR

registers with the new values. Every time the segment changes you have to use the far branch

even if you branch from address (n)FFFFh to (n+1)0000h. This is because the program

doesn’t manage the 6 high-order bits of the 22-bit program addresses if you don’t use a far

branch to change the CSR register value. The script file (described in the Development Tools

chapter) allows you to place all your program modules anywhere in a single segment.

The far branch instruction adds only 2 to 4 additional cycles compared to a near branch in-

struction.

Note:

In C language, using the small code model (this means only one 64 Kbyte segment is used), all

calls use local branches. If more than one segment is used, the large code model is required

and all calls use far branches even when branching locally. To avoid far calls in the same seg-

ment, the segment to be called has to be declared as static if it is not called from another seg-

ment.

3.1.7.1.1 Segment and Offset in Assembler Mode or C Language

The Offset represents the address in the segment with 16 bits. If the Segment and the Offset

are known, the following syntax is used for the far branches:

jps

segment,offset

; 6 bits + 16 bits

jps

symbol

; 22 bits

calls

segment,offset

; 6 bits + 16 bits

calls

symbol

; 22 bits

calls

(R),(rr)

; 6 bits + 16 bits

calls

(r),(rr)

; 6 bits + 16 bits

rets

; 22 bits

The assembly tools accept a set of directives which retrieves the elements of a function ad-

dress or of a label.

The operator SEG (stands for SEGment) allows you to extract the segment number of a label;

similarly, the operator SOF (stands for Segment OFfset) allows you to extract the offset of a

label within its segment.

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