assembly - Commodore PET BASIC 从 $00C2 开始组装什么？

Question

我发现的 PET 的零页内存映射声称零页地址范围 $00C2..$00D9 用于静态数据，例如http://www.classiccmp.org/dunfield/pet/petmem.txt 说:

 RIDATA 00C2        Cassette Temp (64#00AA) read flags: 0=scan,
                    1-15=count, $40=load, $80=end of tape marker
 RIPRTY 00C3        Cassette Short Cnt (64#00AB): counter of seconds
                    before tape write / checksum
 PNT    00C4-00C5   Pointer: Current Screen Line Address
 PNTR   00C6        Cursor Column on Current Line
 SAL    00C7-00C8   Pointer: Tape Buffer/ Screen Scrolling
 EAL    00C9-00CA   Tape End Addresses/End of Program
 CMP0   00CB-00CC   Tape Timing Constants
 QTSW   00CD        Flag: Editor in Quote Mode, $00 = NO
 BITTS  00CE        Cassette Temp (64#00B4): Tape read timer flag
                    =IRQ enabled for Timer 1
        00CF        End of tape read
        00D0        Read character error
 FNLEN  00D1        Length of Current File Name
 LA     00D2        Current Logical File Number
 SA     00D3        Current Secondary Address
 FA     00D4        Current Device Number
 LNMX   00D5        Physical Screen Line Length
        00D5        4.80: right side of window
 TAPE1  00D6-00D7   Pointer: Start of Tape Buffer
 TBLX   00D8        Current Cursor Physical Line Number
 DATAX  00D9        Current Character to Print

但是，查看 ROM 反汇编，可以找到地址 $00C2 跳转到的位置，例如 http://www.zimmers.net/anonftp/pub/cbm/firmware/computers/pet/d/rom-1.html#C70A :

 C70A  4C C2 00             JMP iC2       

在启动 PET 后查看从 $00C2 开始的反汇编，我可以看到看起来合理的代码:

.C:00c2  E6 C9       INC $C9
.C:00c4  D0 02       BNE $00C8
.C:00c6  E6 CA       INC $CA
.C:00c8  AD 00 04    LDA $0400
.C:00cb  C9 3A       CMP #$3A
.C:00cd  B0 0A       BCS $00D9
.C:00cf  C9 20       CMP #$20
.C:00d1  F0 EF       BEQ $00C2
.C:00d3  38          SEC
.C:00d4  E9 30       SBC #$30
.C:00d6  38          SEC
.C:00d7  E9 D0       SBC #$D0
.C:00d9  60          RTS

这个区域是做什么用的？把这个程序组装到这个区域的代码在哪里？这段代码应该做什么？ (好像是在 $0400 开始的区域扫描 : 和 字符？)

Answer 1

它是 BASIC 解释器循环的一部分。它读取标记化的 BASIC 程序的一个字节，如果它是冒号或零字节，则设置零标志，如果是数字，则清除进位。您可以在 address C6B5 处的解释器循环的主要部分中看到它的使用。

我不知道为什么这个例程被放在零页。使用 LDA $0400 比使用 LDA ($C9),Y 快一个周期(或很少两个)，但我看不出它实际上有多大区别。

我还应该注意到，您正在查看的 ROM 反汇编似乎适用于 BASIC 1.0 ROM，而您引用的内存映射适用于 2.0 和 4.0 版。

以下是 Sheldon Leemon 的 Mapping the Commodore 64 对等效 C64 例程的描述:

115-138   $73-$8A   CHRGET
Subroutine: Get Next BASIC Text Character

...

CHRGET is a crucial routine which BASIC uses to read text characters, such as the text of the BASIC program which is being interpreted. It is placed on zero page to make the routine run faster. Since it keeps track of the address of the character being read within the routine itself, the routine must be in RAM in order to update that pointer. The pointer to the address of the byte currently being read is really the operand of a LDA instruction. When entered from CHRGET, the routine increments the pointer by modifying the operand at TXTPTR (122, $7A), thus allowing the next character to be read.

Entry at CHRGOT (121, $79) allows the current character to be read again. The CHRGET routine skips spaces, sets the various flags or the status register (.P) to indicate whether the character read was a digit, statement terminator, or other type of character, and returns with the retrieved character in the Accumulator (.A).

...

As this is such a central routine, a disassembly listing is given below to provide a better understanding of how it works.

115 $73   CHRGET  INC TXTPTR   ; increment low byte of TXTPTR
117 $75           BNE CHRGOT   ; if low byte isn't 0, skip next
119 $77           INC TXTPTR+1 ; increment high byte of TXTPTR
121 $79   CHRGOT  LDA          ; load byte from where TXTPTR points
                               ; entry here does not update TXTPTR,
                               ; allowing you to readl the old byte again
122 $7A   TXTPTR  $0207        ; pointer is really the LDA operand
                               ; TXTPTR+1 points to 512-580 ($200-$250)
                               ; when reading from the input buffer
                               ; in direct mode
124 $7C   POINTB  CMP #$3A     ; carry flag set if > ASCII numeral 9
126 $7E           BCS EXIT     ; character is not a numeral--exit
128 $80           CMP #$20     ; if it is an ASCII space...
130 $82           BEQ CHRGET   ; ignore it and get next character
132 $84           SEC          ; prepare to subtract
133 $85           SBC #$30     ; ASCII 0-9 are between 48-57 ($30-$39)
135 $87           SEC          ; prepare to subtract again
136 $88           SBC #$D0     ; if < ASCII 0 (57, $39) then carry is set
138 $8A   EXIT    RTS          ; carry is clear only for numeral on return

The Accumulator (.A register) holds the character that was read on exit from the routine. Status register (.P) bits which can be tested for on exit are:

Carry Clear if the character was an ASCII digit 0-9. Carry Set, otherwise. Zero Set only if the character was a statement terminator 0 or an ASCII colon, 58 ($3A). Otherwise, Zero Clear.