Questions

MSB vs. LSB

• p.5: ... words are addressed by the location of their lowest numbered byte
• p.47: prog 2.2: "LDA three" and storage directive "three WORD 3" ... seems as MSB
• What is lowest numbered byte? Is this LSB?
• Decision: We use MSB!

Register numbers

• p.5: PC=8, SW=9 due to compatibility with XE
• Why is number 7 unused?
• Number 9=10001b does not fit into 4 bits, e.g., in instructions ADDR, ...
• Better would be PC=7, SW=8

Multiple sections with the same name

• Is this allowed?
• No. Since, the CSECT format is: <section_name> CSECT and labels are unique.
• However, the assembler could easily be extended to support section reentry.
• Decision: by the book, no section reentry.

Addressing resolution

• p.59
• first try PC-relative, then base-relative
• if neither is applicable programmer must use format 4 (extended)
• So when direct/absolute addressing can be used? Only for absolute symbols?
• Can we after PC-rel and base-rel have failed use direct? See also next point.

Direct/absolute addressing in format 3 and SIC format

• Renders the code unrelocatable.
• In particular, it is unrelocatable only in the range of 12 bit displacement.
• Should we use this mode?

Immediate with negative operands

• e.g., LDA #-1
• use of direct/absolute addressing
• however in simple and indirect operand gives the absolute address, thus it is unsigned
• can in the case of immediate addressing the operand be signed.
• Decision: the assembler treats immediate as signed, all other as unsigned

Directive ORG value

• p.73: "value is a constant or an expression involving constants and previously defined symbols"
• Can we use forward reference? At least to absolute symbols. Yes?
• Thus, expressions consists of constants and symbols (absolute or (relative) previously defined)

Directive ORG and length of code?

• How does one calculate code length if it contains ORG:
  • so there is a large non-assembled gap, or
  • there ORG address is before start address (we have negative code length)
• See also p. 74 for a discussion on ORG (without parameter) used for resetting to the previous value.
• Decision:
  • At the end of every section we implicitly issue ORG (reset the locctr).
  • Then we calculate the length.
  • Consequently, we ignore all the "ORG address" parts.